Why did American Abolitionists believe slavery to be immoral?

Let's hear it from the most prominent abolitionist of the time, who wrote the declaration of beliefs for the Abolitionist movement in 1830 and was the publisher of one of the oldest abolitionist newspaper The Liberator, William Lloyd Garrison.

On July 4, 1854, Garison publicly burned a copy of the Constitution, condemning it as "a Covenant with Death, an Agreement with Hell," referring to the compromise that had written slavery into the Constitution.
wikipedia

In his 1854 "No Compromise with Slavery":

I am a believer in that portion of the Declaration of American Independence in which it is set forth, as among self-evident truths, "that all men are created equal; that they are endowed by their Creator with certain inalienable rights; that among these are life, liberty, and the pursuit of happiness." Hence, I am an abolitionist. Hence, I cannot but regard oppression in every form – and most of all, that which turns a man into a thing – with indignation and abhorrence. Not to cherish these feelings would be recreancy to principle. They who desire me to be dumb on the subject of slavery, unless I will open my mouth in its defense, ask me to give the lie to my professions, to degrade my manhood, and to stain my soul. I will not be a liar, a poltroon, or a hypocrite, to accommodate any party, to gratify any sect, to escape any odium or peril, to save any interest, to preserve any institution, or to promote any object. Convince me that one man may rightfully make another man his slave, and I will no longer subscribe to the Declaration of Independence. Convince me that liberty is not the inalienable birthright of every human being, of whatever complexion or clime, and I will give that instrument to the consuming fire. I do not know how to espouse freedom and slavery together.

This is a short synopsis of a really great speech, the entire speech is worth reading and is at the link above.

NOTE This was an attempt to answer the question as originally written:

Someone already asked when and how people began to consider slavery immoral on stack exchange, but no one has asked why. So that is my question; WHY did people in the United States and Europe start to think slavery was immoral after doing it and profiting from it for a long time?

The question has since been edited.

It's a very broad question but I will attempt to answer based on England, France and the United States. However, it is worth remembering that the very concept of "morality" is not constant. It has varied over time and between places.

With hindsight, we can say that it was the logical, and almost inevitable, conclusion to the debates around what we would now call "human rights" in England, France and the United States.

In fact, it's not even a particularly modern concept. Even as far back as 1315, King Louis X of France was able to issue a decree stating that:

"France signifies freedom"

and that any slave setting foot on the French ground should be freed!

(However, that decree didn't prevent France becoming an active participant in the trans-Atlantic slave trade. Christopher L. Miller discusses that dichotomy at some length in his book The French Atlantic Triangle: Literature and Culture of the Slave Trade).

Of course, the French peasantry in 1315 didn't enjoy a great deal of freedom either. Indeed, to modern eyes, many of the medieval forms of serfdom often appear indistinguishable from slavery. However, to the medieval mind, this was part of the natural order - decreed by God - and so rather more acceptable). Even slavery had a basis in the Bible, and so was thought to be acceptable in certain circumstances (by those who bothered to give the matter any thought at all).

In the aftermath of the Black Death, which peaked in Europe between 1346 and 1353, Europe had lost something like 45–50% of its population. But the impact was not uniform. Estimates vary, and the figures remain very much a subject of debate, but it is suggested that some regions may have lost as much as 80% of their population, while others as little as 20%.

Either way, this led to enormous shortages of workers, and people suddenly found that their labour had value. Not only could they ask for better pay, but also for better conditions. The very nature of serfdom was changing, and that meant that power was becoming less centralised.

It took a long time, but by the 17th and 18th centuries, people in England and France had developed their ideas of what we would now call "human rights" to the point where they were prepared to challenge, and even execute, their kings in defence of those rights.

Alongside those "human rights" came the protection of the "Common Law", and this is of particular importance if we want to understand why people came to think that slavery was wrong, and even immoral. Sometimes, it is only in the recorded judgements of the courts, and the reasoning that underpinned them, that we can get a glimpse of how people saw their world. This is true for slavery, as we shall see.

It only remained for people to recognise that slaves were also people, to make the case that depriving them of those liberties that everyone else in France and England took for granted was wrong.

The image above is the official medallion of the British Anti-Slavery Society from the late 18th century. The message was clear and powerful. Slaves were people. People had rights. Therefore slavery was fundamentally wrong.

In the case of Shanley v Harvey (1763), the Lord Chancellor, Lord Henley stated:

" ... as soon as a man sets foot on English ground he is free".

He further observed that, in his learned opinion, a negro could take his master to court for cruel treatment. Now, these comments were only obiter dictum, and so not considered binding on subsequent courts, but we can see that the idea that slaves were also people, and therefore also had rights had taken root a generation before the medallion shown above was produced.

Of course, the medallion was a message much more clear, and far more accessible to many more people than an obscure obiter dictum comment by a judge in a court, even when that judge was the Lord Chancellor!

In 1772, Lord Mansfield went further. In the case of Somerset v Stewart he ruled that slavery was unsupported by the common law of England and Wales, saying:

"... [slavery] is so odious, that nothing can be suffered to support it, but positive [statute] law. Whatever inconveniences, therefore, may follow from the decision, I cannot say this case is allowed or approved by the law of England; and therefore the black must be discharged.

Slaves were people. People had rights. Therefore slavery was so odious that it violated the most basic laws of the land. The message could not be more clear. Slavery was immoral.

Now came the inevitable clash between money and morality. It took another generation, but slavery would be abolished throughout the British Empire with the Slavery Abolition Act 1833.

France had already abolished slavery in all its possessions in 1794, only for Napoleon to restore it in 1802. Nevertheless, the principles of liberté, égalité, fraternité would not be denied. She would finally abolish the slave trade in 1818, and abolish slavery in her colonies by the end of the century.

The United States articulated the idea that:

"... all men are created equal"

and that

"... they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the Pursuit of Happiness."

at its very beginning. We should not forget that his was only a few years after Somerset v Stewart. The concepts of "human rights" had crossed the Atlantic with the people who were building the new nation, but it would take time for many to accept that slaves were also people and that those rights proclaimed in the Declaration of Independence also applied to them.

The struggle between money and morality in the United States would take a little longer, but the eventual outcome would be the same. The majority of the people came to accept the inescapable conclusion that those words in the Declaration of Independence, the foundation document of the United States, meant that slavery was immoral.