Does anyone know why Sweeney Todd sings about forgiveness at the end when he knows he is going to kill mrs.lovett for lying to him?

He sings about forgiveness because he doesn`t want to fell guilty for killing people, and at the end, he sings that again to pull Mrs Lovett at the fire.. He was playing nice and he killed her...

He was just singing it to trick Mrs. Lovett so he'd have the chance to shove her in the fire.
I doubt she'd have danced with him if she thought he was still mad at her. I mean, would آپ dance with some lunatic guy with razors that you'd pissed off?