Wolverine is a complex superhero. He is a hero, by definition of the word. Wolverine served as a member of The X-Men, Avengers, and even the Fantastic Four, in Marvel Comics. However, while he is a hero, he is more Punisher than Captain America and has blood on his hands from his time in Marvel Comics.
Few mainstream superheroes kill as much as Wolverine, and even in the X-Men movies, Logan kills without much thought, slashing his claws through anyone and everyone that threatens him or the children at Professor Xavier's school for gifted children.
10 Deadpool
Wolverine killed one of the most popular characters that Fox movies ever introduced from the Marvel Universe. Wolverine killed Deadpool. What is impressive is that, as much as fans love Deadpool, this was almost a mercy killing because fans hated this version of the character.
The death came in X-Men Origins: Wolverine and this was not the Merc with a Mouth, but a new weapon whose mouth was eliminated entirely. Wolverine and Sabretooth fought him and chopped off his head.
9 Mariko Yashida
The death of Mariko Yashida was a tragic one for Wolverine. There have been very few times that Logan found peace in his life, but he believed he found it by the side of his lovely bride, Mariko. Her death came due to an assassin Reiko using poison from a blowfish to kill her at the orders of Matsu'o Tsurayaba.
This death was going to be long and painful, and she begged Logan to kill her to prevent that, and he did. Wolverine was at his most vengeful after that, severing part of the body of Matsu'o every year on the anniversary of Mariko's death.
8 Lady Deathstrike
The death of Lady Deathstrike came in X2. It should be noted that this character has little in common with the Marvel Comics character with the same name. This version of the character is a mutant who works for William Stryker and has similar rejuvenating powers and adamantium claws like Wolverine. In the movie, Wolverine defeated her by injecting her with fluid adamantium, killing her painfully.
7 The Hulk
While this takes place in an alternate world's story, Wolverine did kill The Hulk. This takes place in the Old Man Logan storyline, and the Hulk here was a deranged lunatic. After supervillains took over much of the United States, Hulk got the West Coast and started his own family with his cousin, She-Hulk.
The Hulk Gang was their grandkids and were basically hillbillies who killed anyone they wanted with no one to stop them. The problem came when they killed Logan's family. Logan kills Hulk's grandkids and then fights the big green monster. Hulk kills and eats Logan, but then the mutant rejuvenates inside Hulk and tears his way out, killing Hulk once and for all.
6 Omega Red
Omega Red was a serial killer from Russia who became a cyborg and eventually was one of Wolverine's most prolific villains. He had a lot in common with Wolverine as his armor was carbonadium, which was the Soviet's version of adamantium.
He then absorbed life from those he touched to remain alive himself. The two seemed to always fight to a stalemate, but Wolverine finally killed the villain by using the Muramasa Blade, which nullified Omega Red's healing factor.
5 Jean Grey
One of the most hated X-Men movies of all-time was X-Men: The Last Stand. This movie did a little too much, bringing in the mutant cure storyline and then adding the Dark Phoenix story to boot. There were several changes to the story, the worst killing Cyclops offscreen, thereby eliminating his relationship with Jean Grey from the finale. Instead, the film had Wolverine step up and kill Jean in a sort of mercy killing, stopping the Dark Phoenix's reign of terror.
4 Daken
Wolverine had a son, and that boy grew up to hate his father. Daken was someone that Wolverine had no idea existed. Born in 1946, Daken's mother was killed, and the baby cut out of her womb and given up to be raised by a wealthy Japanese family. However, his new family never cared about him, and he eventually murdered his baby brother.
Daken went on to become a version of Wolverine when he came hunting for his dad. After years of fighting, father and son battled to the death. Wolverine apologizes for not being there for his son and then drowns him.
3 Sabretooth
In some fun Marvel Comics trivia, Sabretooth had a hobby. He shows up every year on Wolverine's birthday and attacks his longtime foe. This is just for fun, as both men always stopped short of killing the other. For Sabretooth, it was just to remind Logan that he could never actually win a fight against him. That all changed thanks to the Muramasa Blade once again. This time, Logan chopped off Sabretooth's head with it, killing his rival once and for all.
2 The X-Men
The Old Man Logan storyline started with a tragedy, one that forced Logan to sheath his claws and walk away from for years. As a member of the X-Men, Wolverine always protected the students and laid his life on the line many times over. At the start of Old Man Logan, villains attack the X-Men, and Wolverine is there to fight them. He ends up killing all the villains, only to learn that Mysterio cast an illusion and Logan ended up killing every member of the X-Men, on accident.
1 A Teenager
Possibly one of the saddest moments in mutant history came in the pages of Ultimate X-Men #41. Not all mutants are good or bad. Some are victims of their powers and would do anything to be normal. This was the case with an unnamed teenage boy who developed his mutant powers, which sent out a toxic secretion that burned anyone to death around him. 265 people died, including everyone he loved. Wolverine hunted the boy down and executed a mercy killing so the teenager would never feel guilt for killing anyone again.