Here's my situation: In a fight with a group of vampire thralls, the party's wizard got caught in a corner and was being savaged by vampire bites, his max HP dropping from 24 to 11. They fended off the vampires, but the wizard was at 3hp (He refused to be healed by the cleric due to his character's hatred of religion and gods). He activated a trap collapsing the temple, and ended up getting hit by a falling chunk of stone ceiling, taking 15 damage (the rock rolled better than any of the vampires).
Now, the wizard is reduced to 0 hp, with 12 damage left over. The cleric's player says that exceeds the wizard's current max hp of 11, causing insta-death. The wizard's player argues that the death threshold for negative HP isn't affected by max-hp-reducing spells, claiming that would make those kinds of spells more powerful than intended.
I have stories planned in either case, but I'd rather be certain that I'm following the rules.
Is the threshold for instant death based on current max hp or normal max hp?