I do NOT know any of the people involved in this tragic loss of life, nor do I know any of the facts. That will be left to the court of law if there is justice.
However, I do know that in domestic violence relationships, threats of suicide are on the continuum of violence. At the end of the continuum is death by homicide/suicide.  
The perpetrator often uses the threat of killing themselves as a means of power and control.  Think of how many times (too many) that whole families die at the hands of a "loved one". 
This young man certainly needs help and needs to be held accountable for his actions. It's too late to help the young woman who lost her life trying to stop him. 
Too many times there are murder/suicides, in this case one person died and one person is still alive. The person who pulled the trigger caused the death of a human being. 
Will there be any justice for the victim ? 
Condolences to the family of the young woman !