Being rude to a player generally doesn't help them improve.
There's a decent chance a player is doing something wrong because he doesn't know any better, and he might never figure it out because other players would berate him rather than try to educate. Even if you're trying to offer advice, if you come off even a little bit as rude then the other player is very likely to ignore anything you have to say.
Just last week I had an LFR where a tank went the wrong way and one of the guys started calling him names and suggesting we kick him, but he just didn't know any better. The name-caller never bothered to clarify why the tank was a "stupid noob" or why that way was wrong, which doesn't help the tank get any better or understand what he had done wrong. This is exactly the sort of toxic behavior that can drive people away from online multiplayer experiences.