Relative to other drugs, that is. In a CrookedTimber symposium on Steve Levitt, I offer a few speculative and possibly false hypotheses:
1. Heroin and pot make you sleepy. Crack gets you riled up.
2. Crack was a new drug when it hit the market. Gangs were competing to hook new buyers. This is a far more violent activity than serving established drug clientele.
3. In dollar terms crack was a "bigger" drug than ever before. The gross and the profit margins were bigger. The resulting turf wars over profits led to murders. It is not worth killing people over a few marijuana sales. (Yet still I find this puzzling. Falling prices have taken profits out of the market; the gangs must either have had an extraordinarily high discount rate or they behaved irrationally in killing each other. In the latter case we have no economic explanation at all for the hike in crime.)
4. Perhaps you buy other drugs from your friends, but you buy crack from dealers. (Most people get Ecstasy from their friends, and this market is not very violent.) The new question is then why this might be. Could crack somehow require less personal certification from trusted acquaintances?
Here is my full post. I have opened up comments for your ideas. Contributions from economically literate and (previously) violent crackheads are especially welcome.