If the lump is in the cat's fur, it is just a knot. If it's on the skin, it might just be a wart or a skin tag. But if it's under the skin, it could mean something serious. You really need to get your cat to a vet straight away.
There can be many different kinds of lumps, and the diagnosis will depend on things like how firm the lump is, how long it took to grow, whether it causes pain, whether the cat has a fever, whether...
Well all cats have different personalities but yes, some cats are mean.
However, cats do not want to be mean. It's not like they are trying. Usually cats are mean if they had a bad past like an owner...