Out of spite, I'm going to tell a girl that her boyfriend is badmouthing her.
Why would you want to tell her?
Because I want to be a mean girl. Then people will respect me.
Inserting yourself unnecessarily into other people's relationships is petty, especially if you're doing so out of spite. You should rethink your plans as being a mean girl is not a good thing.
It's fine. I think he's a bad person anyway and I won't be taking it too far.
If it's not true that's he's badmouthing her then it's still wrong to lie. You can tell her the truth about how badly he behaves if you want to stop her from being his girlfriend.