I am sure he did not intend to kill him, but probably just snapped at the sight of what was happening. Who wouldn't..it is messed up.
I have never understood what would provoke this kind of action upon a child!!!!
He may not have intended to kill him, but I do not think that "victim" is the right wording here if this is really what was happening. The "Victim" is the girl and her family, and will be forever.
At least he will not be doing this, or worse, to anyone elses daughter after he is released in a few years.
If he did indeed catch a man molesting his 4 year old daughter, I don't think it matters either way.
Temporary insanity in full effect.