Is there any documentation that indicates the Aztecs were as violent as they are made out?

Injecting subjective, modern-day judgements on ancient civilizations like calling them "bloodthirsty" is just bad-mouthing ancient civilizations and not objective historically. If your goal is to look at ancient civilizations and just insult them or consider them inferior because their customs are denounced by your own civilization, you will have a hard time understanding history.

To correct Swamp Yankees answer, Aztecs were not "like" Mayans, they were Mayans. The Aztecs were a Mayan people that invaded and conquered central Mexico. Like all Mayans they believed that the god of the Sun needed to be propitiated by the sacrifice of human blood. Mayans often cut themselves and drained the blood into bowls for the purpose of sacrificing their own blood. One type of blooding involved thorns threaded on twine which they would pull through their own tongues to draw blood.

When the Mayans conquered the Mexicans they sacrificed as many as they could to the Sun, sometimes in large ceremonies where hundreds or thousands of prisoners had their hearts cut out and their blood drained in sacrifice to the sun.

As far as violence is concerned, the Aztecs were "violent" and were proud of it. Being a warrior was a privileged position in society and the Mexicans held the Aztecs in absolute dread. Not only the Aztecs but many of the surrounding Mexican tribes were very warlike and violent. For example, when Cortes was marching to Tenochtitlan (Mexico City), his force was attacked by a tribe called the Tlaxcalans who were especially warlike. In one battle, these men kept attacking the Spaniards until the men of Cortes were fighting in a pool of blood that came up to their knees and the number of bodies was in the thousands.

One of Cortes' lieutenants, Bernal Díaz del Castillo, wrote about a book about the march to Tenochtitlan which describes in detail many of the Aztecs and Mexicans whom they encountered, so if you are interested in their attitudes, you might read his book.

Most of what we know about human sacrifices among the Aztecs is known from post-conquest codexes such as Ramirez Codex, Codex Tudela, or Codex Magliabechiano, written by baptized Christian Aztecs.

There is no comprehensive and even quantitative data from any non-Christian source. It is known that Franciscan bishop Juan de Zumárraga burned all pre-Christian Aztec books that could shed any non-biased light at the pre-Christian Aztec customs.

While, as the other answer says Mesoamerican civilizations did not shed much more blood than European ones, they did it in a different way.

Aztecs sacrificed random people, Europeans executed scoundrels.

E.g., if you are a random law-abiding yeoman in England, chances are the Law will not come after you. However, a random guy in an Aztec-subjugated village can easily become a sacrificial tribute.

IOW, people find it easier to associate themselves with the victims of the Aztec cults than with victims of the European justice system.

PS. This is somewhat similar to my other answer.

The closest literary source I have encountered that involves a perspective of these Native Americans is the journals of Bernal Diaz. In the book Victors and Vanquished you can find several of his eye witness accounts that seem to be somewhat unbiased.

From what I have read, these three tribes were indeed "bloodthirsty" but not in the sense of a serial killer. They seemed to believe that the consumption of a superior peer's blood and organs would inevitably improve their well being, agility, intelligence, what have you.

First of all, there is an attempt by some current historians to use cultural/moral relativism when it comes to Europeans and Mesoamerican cultures in this time period. They'll attempt to equate things like the Spanish Inquisition (if they're on the anti-religious political left) or modern day abortion (if they're on the religious political right) with the human sacrifice practiced by the Aztecs. This "noble savage" or "romantic racism" train of thought attempts to obscure and downplay the brutality found in certain, non-European, cultures, such as the Aztecs.

Some of this romanticization and obfuscation was in reaction to earlier historians who regarded Aztecs as uncivilized savages, which was also false. The Aztecs were both quite civilized and quite brutal.

The truth of the matter is that the Aztecs weren't rivaled in the deliberate wholesale killing of civilians until the 20th century by the Nazis. While there is some debate over the scope, even the low estimates far outstrip what the Europeans or Asians ever did during the same time periods.

Culturally, it was a culture of fear, the common result of an authoritarianism. The common people lived in constant fear that they would be next up upon the pyramid of death. They could be whipped up into a bloodthirsty frenzy being motivated by self-preservation and group psychological dynamics.

So, yes, the Aztecs were a violent society. While individually, the everyday people were probably just typical people for the most part, as a group, they are easily one of the most internally violent and bloodthirsty cultures to have ever existed beyond the tribal level.

Your question is framed oddly. Human Sacrifice, and lots of it, were common in pre-columbian Mesoamerican cultures: not just the Aztecs, but the Mayans and a bunch of others, too. No apocalyptic justifications apply, they just killed a bunch of people for their religious rites. It got so bad, the client states of the Three Part Alliance, who had to supply the bulk of the sacrifices, despite massive depopulation due to disease, basically revolted as soon as Cortez showed up. (Showing once again, it's better the devil you know than the one you don't...)

On the other hand, as Charles C. Mann points out in his book 1491, pre-modern European cultures were just as bloodthirsty, per capita, as the mesoamerican cultures: they called them executions rather than sacrifices. While some of these executions were of murderers and other scoundrels, a lot of them were for petty political and religious reasons - exactly the sort of thing that gets you nominated for this year's sacrifice in Tenochtitlan. On page 134:

Between 1530 and 1630, according to Cambridge historian V.A.C. Gatrell, England executed seventy-five thousand people. At the time, its population was about three million, perhaps a tenth of the Mexica empire. Arithmetic suggests that if England had been the size of the Triple Alliance, it would have executed, on average about 7,500 people per year, roughly twice the number Cortés estimated for the empire.