It was stupid that the MC killed the two masters; he could have just incapacitated them, but all he's doing is eliminating support for the girl he had sworn to protect. What it boils down to is selfishness: he wants the assassin to live so he can "win her heart" and complete one of the tasks given by the love god's app. The MC brutally murdered the assassin's father, how could he ever think she'd fall in love with him? It must be hentai logic.