unfair treatment

Is California an "at will" state? Is it legal to have fellow employees "spy" on other employees? Can you fire someone because you don't like them, even if they do their job well?

At will state

Yes, California is an “at will” state. It is legal to have employees watch each other as long as it does not amount to a violation of privacy. But watching someone’s job performance is not illegal.

And unfortunately, in an “at will” state, it is legal to fire someone because you don’t like them, no matter how well they do their job. In fact, no reason is need to fire someone.