I’m seeing someone who is not exclusive with me and lies to me. Do you think when I cut things off with him, to use the phrase, “I don’t think you deserve me.” or “you’re not good enough for me.”, would be entirely too rude?

Sweetie, you’re not being rude enough.

Short of giving his best friend a blowjob, there’s very little you could do to end the relationship that would be too rude.