Been in the process of doing this to one of my characters. He's pretty stubborn with his morals and hasn't been up for much violence (outside some bare minimum for self defense). I've been making him go through a lot of crap in his game and had some pretty consistent negative CR with someone who likes pushing his buttons. He'll eventually snap at this rate and lash out.

Like the above anon said, it depends on who you're talking about and how stubborn they are/their reasons for not being violent. But usually, I'd say it's something that's built up, not something that just happens. ...Unless something really horrible is going on.