Most superheroes have a clear idea of what's right and wrong and know what they have to do in order not to cross the line. However, considering how many dangers they face on a regular basis, it doesn't always end well for them. Some Avengers turned evil against their will, usually because of an external influence.
Others were forced to it by their difficult life situation or by things outside their control. Whatever the cause, the end result was usually the same - the heroes started killing others, sometimes even those whom they once considered friends and allies. The good news is that most Avengers continue to do what's right, but there has also been a fair number of those who turned evil in the past.
6 Bucky Barnes
Even though he was never openly proclaimed a part of the Avengers in the MCU movies, Bucky Barnes was a member of the team in the comics and even took on the mantle of Captain America for a time after Steve Rogers' death. Of course, he's one of the exceptions as he turned evil even before becoming the Avenger.
Bucky was a hero who fought alongside Steve Rogers in World War Two. But when he was captured and brainwashed, he became an assassin known as the Winter Soldier. It took him a long time to become free and after that, he had a hard time dealing with the things he had to do while he wasn't in control of himself. Yet despite his unfortunate past, whenever Bucky had to make a decision on his own, he stood on the side of the heroes.
5 Captain America
Steve Rogers' transformation was a major point of controversy in the comics - when he turned out to be a secret Hydra agent by saying "Hail Hydra" in the comics. Captain America's fans couldn't understand how he could have willingly turned sides like this since he spent his whole life fighting against Hydra and its leader, Red Skull. The answer was that he didn't do it willingly.
Instead, Hydra used the sentient Cosmic Cube which manifested as a young girl named Kobik, and persuaded her to rewrite Steve Rogers' history, making him a servant of Hydra right from the start. So even though Rogers had his own secret plan and wanted to get rid of Red Skull, it wasn't because it would help the Avengers but because he wanted to take control of Hydra for himself. The heroic Steve Rogers eventually made a reappearance, but the evil version survived as well, for the time being.
4 Daredevil
Even though he has yet to join the Avengers in the MCU, Daredevil worked alongside the team multiple times in the comics. Considering how difficult his life has been and how much he has lost, his decision to help the world is even more admirable in this context. Yet even Daredevil had his weak moment when he turned evil.
It happened due to a demon who possessed him. Initially, even the possessed Daredevil continued to fight evil in his city, something he's been doing for a long time. But he chose more drastic means to combat crime, started killing people (such as one of his greatest enemies, Bullseye), and eventually turned against his friends as well in the comic book storyline Shadowland.
3 Hulk
Just like multiple superheroes, Bruce Banner didn't turn evil in the main storyline. Instead, it happened in a possible future in the storyline Future Imperfect. After surviving several nuclear wars, Hulk became even more powerful thanks to absorbing the strength of the nuclear blasts.
It negatively impacted his mental health, though, and he became dangerously unhinged. And since most of the other superheroes were dead in this post-apocalyptic future, there seemed to have been no one left who could stop the Hulk, now known as the Maestro, from doing exactly what he wanted.
2 Scarlet Witch
Wanda Maximoff had a difficult life. Born as the daughter of the infamous antihero Magneto, her first comic book appearance marked her as a villain. However, just like her fellow Avengers Black Widow or Hawkeye, Scarlet Witch didn't remain a villain forever. She parted ways with the Brotherhood of Mutants and joined the Avengers instead. Her transformation arc didn't end there, though. After she lost her children, Wanda attacked the other Avengers because she blamed them for the death of her sons. As a result, she killed Hawkeye, Scott Lang, and Vision.
Another villainous act Wanda committed in the comics was when she erased the powers of about 90 percent of mutants. Wanda's villainous era has made it into the MCU as well - first when she initially fought alongside Ultron and then when she took control of the citizens of Westview and then tried to kill America Chavez as well as Doctor Strange in the recent movie Doctor Strange in the Multiverse of Madness.
1 Zombie Avengers
Even the noblest and bravest of heroes were unable to control their dark impulses when they were infected and turned into zombies in the series Marvel Zombies. Suddenly, the Avengers focused on not protecting the world but eating everyone remaining. And since there wasn't enough food, they also clashed with each other and fought for the remaining food resources.
It's difficult to pinpoint who was the worst of them all, but, for example, Hank Pym stands out as he captured T'Challa and then kept him alive in his basement while he continued to eat pieces of him. The series was extremely dark at times and showed once again that zombies have no mercy for those who cross their way.