No, it is technically against FB rules to do so, but it is not illegal and lots of people do. There are a lot of Pets with FB pages LOL. Just remember that if they want to, they can always remove the account if they want to since they say you can't. (not that they ever do unless you try to say you are actually a celebrity or something.. I wouldn't try that, they would not only cancel the acct but the person you pretend to be could technically sue you for identity theft or impersonation!)
Not unless you are doing it for illegal purposes such as damaging someone's reputation by spreading lies, for example. If you are a creep, it could create problems for you when applying for a job or obtaining a security clearance years later. If you create a fake account, it had better be of a fictitious person or an alter ego of yourself.
Why waste your time? Are you a predator of some sort?