I think one of the reasons the pair are remembered today is because they were both relatively high functioning individuals. They got picked on a little in high school but had a circle of good friends, held after-school jobs and went on dates with girls. Though Harris was generally regarded as a nasty smart aleck, unpleasant to be around, Klebold was a little different. Klebold could sometimes come across as charming and shy. His prom date, if I recall correctly, described him as "cute."
So yeah, this means they're pretty much screwed. There wasn't much evidence of insanity before the attack, nothing they could use. Furthermore, they left copious journals and videos about planning the rampage. Both apologized to their parents, demonstrated they knew right from wrong and generally appeared "lucid."
With an insanity defense out, what can they do? See who wins the race to the DA's office, offering to plead to life without parole in exchange for testifying?