The sheer vastness of the X-Men universe and the various timelines and retcon moments make it hard for fans to view any character in a particular way, whether evil or good. Although it might be tough to classify individual characters as good or bad, it is easier to take their actions into consideration and apply the very basic argument of morals and ethics to come to a conclusion.
Given that there are so many telepaths out there who can simply control your mind to force you to do things against your nature, it is sometimes harsh to judge character actions. Nevertheless, there are some instances in which fans are unlikely to forgive.
10 Pyro
Pyro was a student at the Xavier School for Gifted Youngsters. He had the ability to manipulate fire by changing its shape and temperature, but different storylines show him differ slightly in his power. For instance, he could generate fire in the Simon Lasker version but could not do so in the St. John Allerdyce version. He was later brainwashed by another mutant, Mesmero, into joining Magneto’s Brotherhood of Evil Mutants.
Pyro appeared in the 2003 X-Men movie X2 where he is shown to be friends with Iceman and Rogue, but at the end of the movie shifts allegiance to Magneto. In the 2006 movie X-Men: The Last Stand, he faces off against Iceman but is defeated and knocked unconscious.
9 Angel/Archangel
Warren Worthington became a freak to others when feathered wings grew from his shoulder blades. He slowly learned to use his power of flight to help others and later on enlists at Xavier’s school.
During the Apocalypse saga, he has a more antagonistic role and switches sides by joining Apocalypse and becoming one of his Horsemen, thereby transforming into Archangel. In the 2016 movie X-Men: Apocalypse, he fights with Nightcrawler in an airplane and is rendered unconscious
8 Colossus
Piotr "Peter" Nikolayevich Rasputin is a Russian mutant. His powers include changing his entire body into a metallic form, which greatly enhances his physical strength. His character is shown as soft-spoken, honest, and loyal, as shown by several instances where he uses his powers to help others.
He switched sides to Magneto’s after his sister Illyana’s death due to the Legacy Virus, which was created to wipe out the mutant population.
7 Glob Herman
Robert Herman joined the Xavier Institute after the latter was publicly revealed to be a mutant. His body is made of transparent skin due to it being “living wax,” which he can put on fire.
Later on, he joined the Omega Gang, which was created by like-minded mutants to take revenge for human atrocities against their kind. They were eventually defeated by the X-Men and sentenced to carry out various humanitarian tasks.
6 Kid Omega
Quintavius "Quentin" Quirinius Quire joined the academy after Professor Xavier had returned from having prevented a hostile relationship with the mutant haven of Genosha. He was a very powerful telepath as well as a brilliant student while at the school.
He founded the Omega Gang when he convinced a group of fellow students to attack a group of humans as revenge for the crimes committed by humanity against the mutant kind. They even went on to attack Professor Xavier and held him hostage.
5 Rachel Summers
Rachel Anne Summer was from Earth-811 of an alternate timeline. She was the daughter of Scott Summers and Rachel Grey from the alternate Earth, and she had a brief membership with the X-Men when she bonded fully with the Phoenix Force after her mother’s death.
She was brainwashed by the mutant-hunter Ahab, who then forced her to use her powers to track other mutants.
4 Jubilee
Jubilation "Jubilee" Lee was able to generate pyrotechnic blasts from her hands. She shared a strong bond with Wolverine, who convinced her to join the X-Men. As a result of the bio-engineered virus released by a vampire suicide bomber, she gradually turns into a vampire.
She even turns Wolverine into one when he comes to rescue her. As a result, she is kept in a holding cell by Wolverine and Cyclops despite others’ suggestion that she should be killed.
3 Tattoo
Christine Cord was a young student at the Xavier Institute. Aptly named tattoo, she had the ability to display messages and designs on her skin while also possessing the ability to phase through matter.
She later joined the Omega Gang, where they took matters into their own hands to fight for their beliefs on mutant rights. She lost her powers after M-Day. She also attacked Wolverine and Beast, although she was mentally controlled by Cassandra Nova.
2 Cyclops
Scott Summer is one of the most famous X-Men members out there. He can emit Optic Blasts from his eyes, which deal a massive concussive force. He is also one of the first members of the Xavier Institute, but there have been various decisions made by him, which have divided fans on whether he is a good guy or a bad one.
Questionable decisions like choosing the X-Factor over his family, kicking Profession Xavier out of the mansion, using an enhanced Legacy Virus to kill a Skrull fleet, and several others have always kept his motives in question.
1 Dark Phoenix
Jean Grey is another early member of the Xavier Institute and also one of the strongest characters in the universe because of her association with the Phoenix Force.
Despite being a caring, loyal, and nurturing person, she has put those around her in danger several times because of her inability to control the vast amount of energy within her. One of the most iconic instances of the Dark Phoenix Saga was that of her consuming a star, which kills billions of living organisms.