When I pulled the start cord it almost took my hand off. I can turn the blade by hand with some trouble. Does this mean the problem is probably with the piston/ring ? Don't really need the mower was just seeing if I could fix it.

Yes, it could be in the pull start assembly. To make sure remove the spark plug and try pulling on it again. It should turn over with ease.

Could be the flywheel key.
Make sure it has a blade on it.
If not this will cause it to pull back.