Revenge that went He Who Fights Monsters. Even more devastating when during Bob's childhood Adults Are Useless and the law is too incompetent, giving Bob a desire to finally settle justice in his own hands.
After all, experiences, especially past traumas, can be groundwork on how we develop our ideas and philosophy in life.
Alternatively: Bob laughs at the idea that he kills people because of his parents or that people should feel sorry for him because of it.
Bob's backstory is framed as a cautionary tale, not an excuse.
His excuse eventually leads him to commit murder.
Alternatively: One of his many excuses is revealed to be the truth.
Alternatively: Bob's laughter is unconvincing, and he doth protest too much on the subject; even if he chooses not to acknowledge it, he still has issues with his past that inform his actions.
Bob routinely engages in mass murder and world conquest as an adult because the expensive bike he got for his fourth birthday was blue and not red as he wanted, or some other similarly trivial, pathetic and laughable excuse.
Bob's Dark and Troubled Past explains his motivations and methods, perhaps even how his personality and style of relating to others developed and solidified, but everyone (including Bob himself) agrees that it does not excuse his villainy.
Lampshaded: "There must be something really terrible in his past to make him capable of this..."
Invoked: "I want to tell in you in detail why I'm doing this to you... I want to make you understand..."
Bob uses his excuse to make heroes let their guards down.
A hero mocks Bob about his excuse, causing the latter to make a mistake in anger.
Bob brings up his excuse to gain sympathy and a claim to moral high ground in order to get heroes off his back.
Bob brings up an event from his past to shame someone present, who was involved with that same event.
"I cannot be reduced to a simple, trite motivation."
"No, Bob. Your excuse is merely a convenient way of fleeing responsibility."
Discussed: "These guys always have some excuse..."
Conversed: "Do these writers think that having an awful childhood automatically makes you criminal or something?"
Bob's Freudian Excuse, no matter how convincing or painful, doesn't change or is even unconnected to the fact that he has committed numerous irrational, vile and evil actions all on his own initiative as an adult. While we might feel sympathy for what he went through as a child, he should have resolved it peacefully and had no right to commit such actions as an adult, losing his right to our sympathy once he did so.
Bob decides to look-up for psychological help to get through his flaws, and eventually learn how to move on.
In real life, modern theories like operant conditioning and social learning can still explain the effectiveness of a "Freudian" Excuse, while it is shown that neglect does cause lasting brain damage. Who knows, from Bob's perspective he is simply doing his vile actions out of a perceived necessity that was conditioned via receiving punishment/violence for his altruistic behaviour, and is actually good in the inside.
Played For Laughs: Bob brings up his sad past in the most melodramatic way possible. Ben remarks: So that's why you are so emo!
Played For Drama: Bob is pushed to the edge by PTSD. He has trouble believing he could get any better and thinks he and nothing else has no hope. He's considering taking an entire universe with him as he self-terminates and our hero needs to convince him, by any means necessary, that it doesn't have to be.