Having difficulty understanding "the Crucible"

What do the bold parts mean?

“…She thinks to dance with me on my wife’s grave! And well she might, for I thought of her softly. God help me, I lusted, and there is a promise in such sweat. But it is a whore’s vengeance, and you must see it…”

And also, if they say that it is someone’s vengeance, does it mean it is a revenge the person took, or a revenge on the person?