Vocal Unit as pickup lines
I think Levi was actually geniunely serious about torturing Annie because of all the deaths. His violent tendencies and impatience are probably his strongest flaws. Also, if he wouldn't have said any of that, maybe they could've eventually gotten Annie back without her getting away? At one point I associated it with Levi's "I always talk too much" line from after the mission. Maybe he felt guilty about opening his mouth then? But now, I don't think that's the purpose of that scene.