Wow, ok. Not dismissing your reaction -- I mean that sincerely and not in some new age-y, your truth b.s. kind of way. But there's a difference between feeling guilty and actually bearing responsibility in such a scenario.
I just remember my kids when they were teenagers and how perilous that period felt. They were always good, empathetic kids -- did some stupid things, but never anything materially cruel or irrevocably stupid. But I always felt that it would only take the wrong set of friends/influences to lose them -- I used to wonder, what do you do if your underage teenager just flat out refuses to obey? It's not like there are readily available resources or solutions to deal with that.
Fortunately, I never had to find that out. But, what about if your kid is just a flat-out sociopath? While I certainly can find fault with the Columbine kids' parents failure to do a basic thing like surreptitiously search their room from time to time, they took a lot of standard and "reasonable" actions prior to their kids going off.