Personally, I look at it as "is my desire to see someone suffer significantly smaller than the suffering they intentionally caused?" And if the answer is "yes" I don't have to feel bad for it.

Real world example: a close friend of mine's biological mother was abusive to all of her kids in every definition of the term, and it sounds like she was no better of a wife, friend or relative. She died alone in her shitty trailer at the age of 50 after alienating all of her friends and family. There was no funeral, her obituary literally only stated that she existed and who she was related to. And it sounds like it took several hours for her to finally kick the bucket, plus multiple days to find the body. The suffering she endured was orders of magnitude less than the suffering she intentionally put others through, so I see no wrong in feeling happy with her fitting demise