If you humiliate some one in a piece of writing that is published, you become open to a libel action against you. The only defence is that what you have published is the truth. Unlike most situations where the accuser has to prove that the accused is guilty, in the libel laws, the accused has to prove his innocence, the accuser doesn't have to do anything but bring a complaint that his character has been injured in a permanent way, a published piece of work. Slander is different, it is impermanent, the law dealing with spoken statements.
So yes, all writers who aim for publication should deal very carefully in what the say about living people.
Fiction writers also have to be careful, if their story can be linked to someone, however tenuously, they are still liable to prosecution, and - 'any similarity to persons living or dead is coincidental,' is not a defence.