I don't think it's so simple as to written off as some genetic issue certainly and, to be honest with no malice intended, the idea there will be no mental illness at some point in this center is the dumbest thing I've heard in awhile, but I also don't think the "line" between being "sane" and "killing everyone in sight" is fine; why would that be the case? It seems if it really were as precarious as you say, we would have a lot more violence. Only a very small percentage of people become these sorts of rampage shooters.
Wasn't there a time not too long ago when tribes of people would go around raping, pillaging, and slaughtering others just for the hell of it? Because it was fun for them? If our world had respawn, I think life would turn into a sort of valhalla-ish existence for a not small percentage of people, and I bet many would learn about a part of themselves they did not know existed, or had supressed.
Maybe rampage shooters are just people born in to the wrong time period, where in another they would fit in perfectly. The line isn't fine, but they aren't as different as we make them out to be.
