I think rampage is a rather loose term. Let's say someone snaps and stabs 2 or 3 people, would you call that a rampage? Is there a minimum number of victims for these acts to be called a rampage? I think one can't properly answer your question with out defining what you consider to be a rampage.

Two, I'm actually not entirely convinced that the people doing these rampages are actually mentally ill (a certain a portion of them have history of mental illness, but I believe there's a tendency to label an attacker mentally ill when we don't fully understand why they did what they did)

Pretty sure I've violated some rule with this response, but I genuinely feel that these points are relevant to the question.

I am a little unsure of what you are asking. Would you mind clarifying or expanding?