Complex sentence: When he finds out Hisui left of his own volition and is deeply in love with Kokuyo he's very hurt by it, but he wants Hisui to be happy and lets him go, though he vows to hate Kokuyo forever.
Split into simpler sentences: When he finds out Hisui left of her own volition and is deeply in love with Kokuyo he's very hurt by it. But he wants Hisui to be happy and lets her go, though he vows to hate Kokuyo forever.