Killer's Motivation

So it was just some friend of the guy who died, who happened to run into him that day, and then witnessed his (accidental) death. I get him blaming the girl for driving him to suicide/accident, but then the killer starts screaming "I had it all, and you took it away from me! You ruined my life!" Lol, what? How? By inadvertently causing the death of some random dude you bumped into that you knew years before?