Guilt

Sometimes I feel really when I read about the 13 who died and my first thought iß -shrug- it could've been so much worse

Hmmm...yes it could have been worse. So could 9/11, OKC Bombing, all the other school shooters, the Norway Massacre, etc. But tell that to the survivors and victims' families. Thankfully, none of those WERE any worse than what they already were.

Very true but columbine in particular had so many notable errors ( faulty bombs, horrendous police response)

Yes, that's true, and fortunately E & D was completely unaware of the police tactics which I think may have contributed to more lives being saved...either that or the up close, personal nature of murder lost its appeal quickly.

Exactly, it always baffled me how they walked they halls, jiggling locked doorknobs, even making eye contact with some of the people hiding inside, yet didn't even attempt to kill more

Up close, personal kills with a 12 gauge shotgun loaded with .00 buck...the carnage would be repulsive. And to know you inflicted that.

I think they ran on pure adrenaline in the library but when that wore off, I wonder what they thought/felt.