Actually, it was posted that death strike would stack in publish 32:

"Death Strike is now capped at 60 damage if the target moves and 20 damage if the target doesn’t move, with 66% of the damage determined by the ninjitsu skill of the attacker and 33% of the damage determined by the number of tiles the target was tracked for. Chained Death Strikes can raise these caps up to 70 and 30 damage, respectively. "

I think they screwed it up somewhere though. I'm getting hit for 40 to 60 with death strike without moving....