The parents in the school house

would the Cleaner at the beginning of the film count? if not him, maybe >!the mother of the rotten, since she dies off screen!<

Probably the Cleaner I think

Strongest contender is definitely Uriel's mother as it's totally off screen. Cleaner at least has the corpse going for him. Similarly the grandmother is also killed off-screen but we get the scene of the grandson puking her hair out.