I don't think I should give someone back their cat.
Why don't you want to return them to the owner? Why would you not return one's property?
I'm afraid if I give them back it might be dangerous for the cats.
You are concerned about the cats, which is okay if they are really in danger. Try to delve into the matter deeper first to see if that is really the case, and not jump to conclusions. If you are mistaken then the owners will be very sad not to get them back.
Maybe I'm doing this to prove a point that people are heartless animals.
Well, I think you should give back the cat. If you feel like it's been mistreated in anyway, maybe call the authorities to deal with it. If you withhold the cat, isn't that like a kidnapping?