but the point is that killing a +utility person being a reduction in utility is a vital axiom

I don't know if we need have it as an axiom rather than this being a natural consequence of happy people preferring not to be killed, and of us likewise preferring not to kill them, and of pretty much everyone preferring their continued lives to their deaths... The good of preference utilitarianism is that it takes all these preferences as an input.

If preference average utilitarianism nonetheless leads to such an abominable conclusion, I'll choose to abandon preference average utilitarianism, considering it a failed/misguided attempt at describing my sense of morality -- but I'm not certain it needs lead to such a conclusion at all.