The demon in his sword is a seductive one. You've corrupted his mind and bent him to your will so he's yours to do with as you please. He's so warped that he even lets you out of his sword for however long you want even though he knows you'll probably kill people.

He's just so blinded by his love for you that he'll do whatever you ask of him. It's how he nearly lost his position twice because you sent him on a different operation. He doesn't mind, as long as he gets you at the end of the day.