the murderer is the murderer. now, there may be factors involved that drove the person to murder, but they don't justify the murder. And they certainly don't get to blame the MURDER VICTIM, or their parents, for murder.
but I can see you're really fixated on this logic, please, don't let the people who bully you drive you to murdering someone, even if, apparently, you've put A LOT OF THOUGHT into this entire chain of events.
No one's going to give a fuck who bullied you once you start shooting mate.