Thanks for trying to take care of this barn cat! A bloated belly in a cat can be from numerous things: pregnancy (if female/spring time), different types of parasites (which may not be "covered" in your dewormer), cancer, or even FIP - feline infectious peritonitis. This last one FIP is potentially contagious to other cats, and results in abnormal fluid to accumulate in the abdomen. It also results in a really high blood protein level.
You can try a more broad-spectrum dewormer that covers everything (like Drontal for cats or Panacur). If that doesn't improve things, I'd really recommend a vet exam to make sure there's no abnormal fluid in the abdomen from FIP - otherwise, you may have a lot of really sick cats at your barn for the next few years!
Giving the wrong type of dewormer can, yes, kill an animal. If you use one that could potentially kill heartworms, and your pet is positive for heartworm disease, it can result in an anaphylatic reaction (potentially resulting in death).
Generally, the OTC dewormers you buy don't typically cause this. That said, you should consider bringing him to a vet for Drontal (a full dewormer).
I saw worms again Just this am as I let him out. I dont think I'll even bother worming my pup with the same stuff. Ive heard if a cats badly wormy that deworming can actually kill them- is this a wives tale? or an over doing it with the wormer by giving to much of it and to often?