Sarcasm is always nasty - it's using irony, which in itself is merely a harmless way of skewing the way you say things (or the way you see things), and using it as a weapon.

From the Greek sarkazein - "to tear, gnaw flesh lige dogs, to bite the lips, to sneer, taunt", from sark/sarx , flesh" - as my dictionary puts it eloquently.

"No dark sarcasm in the classroom" as Pink Floyd also put it eloquently.