A person would do such a thing because he felt completely alone and powerless, I would assume. The final act of shooting himself is the last, sad attempt to control his situation, instead of being at the mercy of others and under their control. I think that is usually the issue driving such people to commit violent acts.

As such, it's their misperception of their own situation, as well as a reaction to outer conditions which they find painful in some way. In attempting to deal with their own pain, they cause pain to many others.