what would you do if you thought certain people were laughing and getting off on your unhappiness/misfortunes

Those kind of people are best avoided and don’t give them the opportunity to know your business or how you are feeling. They are miserable twisted people to laugh at people who are suffering and they get some kind of perverse energy from it.

I do think that.

I'd try to find out if someone else get that impression of this person too or if it's just me. If I found out it was true I would erase them from my life.

in such a case

I would say to myself
it is not all about you paperlace
the world does not revolve around you
you are not even on anyone's mind as much as you think

Run their bodies through a wood chipper and feed their remains to reindeer