Fair points. I already started mitigating this by disabling friendly fire for players with -1 score (that is, as soon as a player has team killed once and made no valid kills, they won't be able to teamkill again until they make one legit first).

Instead of hurting players (-20 health) with friendly fire, I thought it would be fun for teamkillers with -1 score to actually give their teammates +1 health by shooting them. I think some players have started using it as a way to have a "healer class", which is kind of a fun emergent behavior.