{I'm curious, since you suggest that Hitler was somehow "legal", do you think a dictatorship might happen in the US as well? Do you think those odds are big? Hitler wasn't really "legal"}

To address your question: Aside from this question being largely based on wrong assumptions, it also shows a somewhat superficial understanding of the workings of a Dictatorship, and moreover, of how a democracy works.

Dictatorships work by being based on fear, and of course, rewards to some. But mostly fear, instilled by brutal and swift punishments.

Dictatorships work just like American slavery worked: Fear, brutal whippings and dismemberment of feet and arms, taking kids away from their mother. Slaves didn't rise up, even though they outnumbered the whites 10 to 1 because they were terrorized. Also, not rocking the boat meant you lived. Plus, nowhere to go. In places where that was possible, the Africans massively disappeared.

Finally, for these type of questions, Wikipedia is your friend.

I hesitate to call fear of death, beatings or imprisonment a moral mechanism. These are what kept Stalin's grip on power.

The basic calculus of an omnipresent secret police force watching for transgressions and the fact that anyone you discussed overthrowing Stalin could turn you in meant that possible dissident elements were completely isolated from each other and rendered impotent. This reached the point of Stalin's savage purges of the military in 1938 that were a large part of the reason that Hitler could have any hope that his invasion of Russia might topple the Soviet state.

Some considerations you may consider.

• Stalin was extremely popular with the people, and after the war he was seen as the leader of the victorious side in WWII. There was a huge personality cult. Any move against him would be very much suspicious of treachery even if formally legal.

• Mafia-style rule. As you know many of the mafia leaders in Russia are of Georgian origin. Georgians have certain talents as to covertly controling people and imposing their authority even outside of formal system.

• As to your assertion that the Politburo members may want dispose Stalin so to save themselves and their families. This is unevident. In fact they even could think his system is just in general. There was a firm principle in the USSR that the government members and their families should not have any privilegies. Even some of Stalin's own relatives were repressed and his son died in a POW camp.

• Your assertion that a simple Politburo vote would be enough to dispose Stalin is doubtful. Most likely one would need a vote of the Central Committee which was a much wider body, much more difficult to involve in any conspiracy and much more influenced by the popular popularity and propaganda. Additionally, Politburo very rarely (if ever) was convened in full composition.

Stalin's first important position was being elected to the Politburo (the main policy-making and executive board) of the Central Committee (the highest body of the Communist Party of the Soviet Union, directing all Party and government activities), in May 1917. He remained a member of the Politburo for the rest of his life.

During the Russian Civil War (1918-1922) and the Polish-Soviet War (1919-21), Stalin was an officer of the Red Army and acted more or less as a general, giving him military credentials.

Between 1917 and 1923, Stalin held posts as People’s Commissar of Nationalities Affairs, People’s Commissar of Workers and Peasants Inspection, Revolutionary Military Council member, and member of the Central Executive Committee of the Congress of Soviets. These posts gave him much political experience, contacts and credibility.

In April 1922, Stalin was selected to be the General Secretary of the Communist Party, a post that gave him control over all party appointments, promotions, and demotions. This allowed him to promote his allies to powerful positions within the party and remove enemies. He appointed only loyal friends to leadership posts in local trade unions, cooperatives, and army units. Not only did these men report directly to Stalin in many cases, but he kept detailed files on them, and on all party members and industrial managers to make sure of their loyalty to him.

Thus, from a legal point of view, Stalin had two basic levers of power: General Secretary of the Communist Party, and member of the Politburo. Using his power from these two posts, he gradually replaced the members of the Central Committee with men who he knew would be absolutely loyal to him. Since a majority in the Central Committee (kind of the Congress of the Soviet Union) had absolute power, as long as he could control that majority he effectively had the power of a dictator.

What do you mean, 'legal mechanisms'?

Putting "how tyrants hold power" and "legal or moral mechanism" in the same sentence is completely missing the point. Stalin didn't have power because being chairman of Politburo, but he was chairman of Politburo because he had power. I'm not completely sure, but I believe that legally the Politburo decisions actually weren't binding on anybody, although they definitely were so in practice. Obtaining and holding power from in-group threats in such situations is accomplished by using actual power, and legal mechanisms are used only as post factum justification of keeping, losing or acquiring power.

A nice example of 'legal mechanisms' is the actual legal head of USSR (and also member of Politburo) Mikhail Kalinin, whose wife Stalin simply had arrested, tortured, and kept in prison for ~7 years, all the while Kalinin legally (theoretically) had much more power than Stalin. That is not an isolated case.

The OP proposed scenario "10 people in room, during a meeting, no arms, no guns, no monsters inside. They were not handcuffed. Anyone could pass a bill and it would have been over." is somewhat possible - but with some clarifications. If anyone there actually proposes such a bill, then the only realistic result is that within the hour one part of Politburo forcibly removes the other part (whoever voted the opposite) from any and all power they hold. The winning party may or may not be the one with most 'votes' - no legal process would be followed unless as a public show when it's already clear what the result will be. The predetermined result could be a surprise to someone (who's being deposed), but not to the one(s) initiating it.

Personal loyalty and control of security forces

You can hold power by properly managing those particular individuals and organizations that may threaten you. Stalin had a close circle of loyal people, and controlled the internal security apparatus. Stalin also favoured pre-emptive strikes, and mere suspicion or risk of betrayal was enough.

A coup cannot be accomplished by 'passing a bill' - passing that bill is an act that is done after the coup in order to legitimize it. One does not simply 'make a vote' if there's no guarantee of personal safety afterwards - if it's not known beforehand that the security forces won't simply arrest everyone passing the bill or that the army will eliminate the local security. You need many allies to make such a coup, but asking for such allies is a clear death sentence unless they're very, very motivated to be a part of the coup.

OP statement "I think police and army would obey the new master, even if a bit reluctantly." is absolutely false. Internal security would obey Beria, and army would obey their generals, no matter what was the legal situation. If a coup was done legally, not with 'actual power', the chekists or soldiers would easily shoot or arrest everyone involved without question - there was no such thing in Soviet practice as legislator immunity. In addition, any legal decisions not supported with actual force can be simply ignored, deleted, rewritten or not communicated to the people, since party and security apparatus had full control of mass communication.

Divide and conquer

The Soviet power trio was based on competition between internal security forces, the Party, and the army. None of them (their leaders or controllers) could obtain or hold power if the other two were against them; and everyone who'd attempt to gain power would be sure that failure means death. Politic manipulations can ensure that the balance stays intact, and that any conspiracies stay small enough so that they can't get a critical mass.

In addition, even a successful coup is very risky. You need strong allies with their own power base, but only one can be at the top. If you want to be the new leader, then you'll get support only if everyone else is clearly convinced that the old leader is much worse for their own personal interests and safety. This is a hard thing to do, and it generally happens only when/if the old leader becoming frail or insane. Replacing one tyrant with another, risking your life for it, and risking the new leader killing you right after the coup to remove a potential challenger... it requires a very strong reason to do so.

Beria

I think it would be extremely instructive to consider the anti-Beria coup. The conspirators discussed the plans in secret and Beria was arrested by Marshal Georgy Zhukov himself. This plan required an absolute devotion of participants since any leak to a Beria agent was deadly. This is why only high-level people were involved - a Marshal(!!) making an arrest (of another Marshal!)

Khrushchev

Another instructive example is the Anti-Party Group which had a majority on Politburo but failed to remove Khrushchev because he claimed (with the critical support of Zhukov) that he can only be removed by the full Central Committee plenary meeting (and he carried that).

Stalin

Stalin did not consolidate power overnight. He became the gensec in 1922 - as a counterweight to the extremely popular Trotsky. He was the junior member of the triumvirate (with Zinoviev and Kamenev) and was not taken seriously by the party. By 1930 he was an uncontested leader - after ridding himself of Trotsky (1926), Zinoviev/Kamenev (1927), Bukharin(1928), Rykov/Tomsky (1929). By that time, Politburo was stacked with Stalin's people who were grateful for their promotion. At about the same time his ex-personal secretary Mekhlis was given control of Pravda and he started to push the notion of "Stalin the genius and all-father". In a few years rank and file were worshiping Stalin.

Now, imagine, say, Molotov and Malenkov wanting to remove Stalin. They need to kill him more-or-less right away after removing him, otherwise the aforementioned rank-and-file will tear them apart on the spot. They also need to fight the overwhelming temptation to turn the co-conspirator in thus currying favor with Stalin. Finally the next day after killing Stalin they risk being killed by their "innocent" colleagues for murdering "the father and teacher".

They also have to think of tomorrow. They remember that Stalin started with killing off those who were once his equals. This means that, say, Molotov must be thinking "if we depose Stalin today, tomorrow I will have to kill Malenkov - but what if he gets me first?" Trying to outmaneuver a colleague in subservience to Stalin appears to be a better strategy than deposing Stalin to face the wrath of colleagues. Remember that de-stalinization was one of the charges that Beria had to face.

The general atmosphere of mutual mistrust was so pervasive in the Politburo, that it is hard to imagine that a noticeable group might form for such a risky enterprise as a plot against Stalin.