"Hi, I'm a nice normal 42 year old male professional. I've got lots of friends who vouch for me. We all came here with these backpacks<BOOM>"
(Friends - who are still alive) "I can't believe he did that. I knew him...not that well, but I knew him. Wow."
Seriously. Our rules are set up to find people who commit repeated acts. Think about rapists ex ceterea; they're repeat offenders. Suicide bombers commit one violent act.