Assad may be lots of different kinds of bad shit, but one thing he is not is genocidal. The "rebels," however, are far more unified in their hatred for those of the religion of the ruling minority - a minority which was often discriminated against in the past.
Does Assad want to kill all Sunnis? I somehow doubt it. Do some of (most of, possibly?) the Sunni "rebels" want to kill all Alawis? You can bet damn sure of it. Do you know that vast majorities have fled some of these cities "liberated" by rebels because they support the government and fear for their lives?
I heard an interview with one such rebel on the BBC the other day. What did he have to say? That when the war is over, they would kill everyone who had been on the other side. Isn't that just super?
NATO is already involved, whether it's publicly stated or not. Other countries (Russia, Iran) are on Assad's side, whether it's stated or not.