Wolverine has always stood out from the rest of his allies on the X-Men for his willingness to use lethal measures when the need arises. It's been an element of the character since his inception and has been explored multiple times over the years. But it turns out that direction for Wolverine was actually fairly controversial among some Marvel creators.
The early direction of Wolverine actually divided opinion between the creators working on X-Men at that time and then-Marvel's Editor-in-Chief, who wanted Logan to never be overtly murderous -- but to always be on the verge of that, even if it went against his character development.
While the other younger members of the X-Men had a strong sense of morality to their actions, Wolverine was content to kill an enemy if the need arose. A notable early example of this difference occurred in Uncanny X-Men #116, by Chris Claremont and John Byrne. While traveling through the Savage Land, a group of X-Men encounters a guard. While Nightcrawler plans to use his powers to bring down the guard without anyone noticing, Wolverine casually sneaks over and is implied to murder the guard. When he reunites with the rest of his shocked teammates, he merely points out that the path is now clear. There's no moral hand-wringing or debate; Wolverine just killed a guy, and to him that's not a big deal.
As explained by Claremont during an interview with the classic comics-magazine Back Issue, "the idea was Wolverine's not like the other X-Men. He is, when the necessity arises, a stone killer. Nightcrawler is not. Storm is not - she has killed, and it's haunted her ever since. Wolverine, if the need requires it, will go out and kill somebody. End of story. Won't think twice about it. This was a bone of contention with (Editor-In-Chief) Jim Shooter." According to various interviews with both Claremont and Byrne -- as compiled by UncannyXmen.net -- Shooter resisted the idea that Wolverine was so easily slaughtering the enemies of the X-Men. Shooter insisted that all the people killed by Wolverine had in fact survived, as he didn't believe a heroic member of the X-Men would be a murderer.
This included the Hellfire Club guards that Wolverine seemingly slew while making his way towards his captured teammates during the height of "The Dark Phoenix Saga." After reportedly hearing Byrne confirm Wolverine's killings at a convention, Claremont revealed that Shooter didn't care for that interpretation. Claremont said, "[Shooter's] feeling was, X-Men don't kill, and he wanted us to establish that all the Hellfire guards and the Savage Land guys were still alive somewhere, they were banged up really bad, but he hadn't killed them... whereas I think both [Byrne] and I felt that it was very important to establish that Wolverine had this inner lethality about him that marked him as different from the rest of the X-Men."
Shooter also insisted that Wolverine be the wild-card of the team, always on the verge of killing someone - even his teammates. Talking with the X-Men Companion, Claremont revealed that "what [Shooter] wanted was for Wolverine to have the capacity to go crazy and kill but never be allowed to kill. He wanted Wolverine to be as much of a potential danger to the X-Men as to other people." One such moment came during Uncanny X-Men #143. Nightcrawler playfully flirts with Mariko in front of Wolverine and Logan slashes at him. This went against what the creators were doing, though. Claremont explained they'd been "toning Wolverine down, making him more rational, the rationale for this being that he could not have loved Jean, could not have experienced X-Men #137... Wolverine's response was to grow up."
This kind of direction for the character especially bugged Dave Cockrum, another early creator who worked on Wolverine, who also spoke to the X-Men Companion. "When he attacked Nightcrawler for kissing Mariko under the mistletoe... Come on, he knows these people are friends; he's not going to do that... I mean, that was no menace. That was apparently done on Shooter's orders, 'Make Wolverine do something crazy.' Personally, I think that was a bad choice. That's all inconsistent with what they've done with him."
Wolverine's ability -- and willingness -- to kill makes him stand out from the rest of the X-Men. It's weird that Shooter thought he could have it both ways, with Wolverine as a dangerous figure but still restrained. But in the end, the intent of the creators ended up being what won through for Wolverine.