Wolverine has one of the longest body counts of any major superhero. What makes it worse is that throughout his life, Wolverine has seen many of his own loved ones impaled on his claws, between multiple love interests, his son, and more. But the biggest kicker of them
all is the fact that Wolverine's very first murder — not to mention, the first manifestation of his iconic claws — happens when he kills his biological father, Thomas Logan.
In all fairness, Thomas Logan is a pretty nasty guy, and violently abuses his other son, Dog. In addition, during his childhood, the future Wolverine (at this time known merely as James Howlett) doesn't even know that Logan is his father, since he's been raised by the wealthy John and Elizabeth Howlett.
James's life comes crashing down when Thomas Logan shoots John Howlett. James responds by popping his claws for the first time, and then slashing up Logan. Though he and his childhood friend Rose quickly flee afterward, hoping to get away from their past, this single event stands out as a defining event in Wolverine's history, even though he spends an enormous chunk of his life unable to remember it.