Seems pretty straightforward.
Apos eats immortals/angels for their essences or memories. Or rather, he eats their timespores. If they're not ripe yet he's not interested so he uses naive young immortals as pawns to trap the older, more delicious ones. He also tortures the ones before he eats them for flavoring I guess.
Rin's older immortal friends have been captured, tortured and eaten so she's next on the menu.