I don't think anything is necessarily off limits in comedy, but Cook (not that I'm too familiar with his work) never seemed the type for offensive humor. I thought he was just... I don't know. Weird?
I don't know how relevant time is. Chris Rock's Bigger and Blacker special aired on HBO on July 10, 1999. I'm not sure when it was actually taped, but clearly, it was less than 4 months after Columbine. And the bit he did on Columbine was HILARIOUS.
Then again, maybe I'm just ok with it because it was actually funny. "What ever happened to crazy?"