I agree with Hyp pretty much entirely here.
If a character decides to forgo his saving throw then he can't change his mind. Spellcraft checks used against spells being cast only help when counterspelling (and that is a ready action). In this case the character would have used his action to "not save". You can't decide to actively defend yourself once someone has already hit you with a sword (to avoid being hit from that blow) - spells work the same way. You are not casting all round for a standard action spell - the casting and the result happen effectively at the same time.
But he doesn't have to say that. A hamless spell works differently than others and the character shouldn't be "forced" to choose. He could be "enticed" to think he has to choose, but if he has been healed before he would "know" that the spell works even if he is unconscious (and can't actively choose to forgo his saving throw).
Also Slay Living requires a melee touch attack. It does not state touch it states melee touch attack. I know it is rather rules-lawery but it is not handled the same as a cure spell (that only requires the touch attack if the recipient is in melee).
I also assume the doppleganger had the spell memorized since he can't freely swap it out. Looks too contrived (at at the last minute) to me.
If a character decides to forgo his saving throw then he can't change his mind. Spellcraft checks used against spells being cast only help when counterspelling (and that is a ready action). In this case the character would have used his action to "not save". You can't decide to actively defend yourself once someone has already hit you with a sword (to avoid being hit from that blow) - spells work the same way. You are not casting all round for a standard action spell - the casting and the result happen effectively at the same time.
But he doesn't have to say that. A hamless spell works differently than others and the character shouldn't be "forced" to choose. He could be "enticed" to think he has to choose, but if he has been healed before he would "know" that the spell works even if he is unconscious (and can't actively choose to forgo his saving throw).
Also Slay Living requires a melee touch attack. It does not state touch it states melee touch attack. I know it is rather rules-lawery but it is not handled the same as a cure spell (that only requires the touch attack if the recipient is in melee).
I also assume the doppleganger had the spell memorized since he can't freely swap it out. Looks too contrived (at at the last minute) to me.