Undead status and "randomly kill" (assassin) effect

This is something that has bothered me for a long time now.  I want the "randomly kill" effect from weapons like the Assassin dagger and Trump to MISS monsters with the undead status set instead of them disappearing and reappearing.  I never understood why this was set up this way, and really makes instant-kill weapons almost useless when undead bosses and monsters reappear with FULL HEALTH.  It's too easy to forget that someone has one equipped...

Does someone know how to make these weapons miss instead?