I am sure you all know the kind. They would be Mean Girls if they had the cajones. But they don't, so they sit back, behind closed doors and computer monitors and say mean things about people they've decided not to like for whatever their reason de jour is. Then they are just sweetness and light in public.
It's cowardly in two ways. 1) they still get to pretend to be nice girls to everyone else and 2) the target has no means of defending against anything they say.
Do they really think people don't figure them out eventually?
How unhappy does one have to be to allow themselves to become this kind of pathetic creature?