She isn't angry that the demon badmouthing her mother as a slut. She's angry at the Demon for thinking her mother is dead, a sinner and is going to Hell to do some "sucking".

fsnfanboy said in comment #2596837:

She isn't angry that the demon badmouthing her mother as a slut. She's angry at the Demon for thinking her mother is dead, a sinner and is going to Hell to do some "sucking".

That's the joke.