( Since he's been asked this a couple of times! )

Remember, this week, time was very slow. I don't think anyone could have a real "instant" death this week so regardless of how he died... maybe it was very slow for him and the killer to experience. Maybe he as injected with morphine so he could fall asleep while he died.

Morphine is used as a painkiller too, isn't it?

Maybe our killer tried to be merciful in the end...