Well im nice to people if i feel they deserve it... so around friends or people i dont really know yet, i like to think im a pretty 'OK' guy, but if someone actively annoys me or they dont get the hint that i dont like them, i stop really caring what they think. That doesnt mean i go and have a go at them, but more.... if i disagree with them, i will tell them. If they do something wrong, i point it out. Its kinda my way of telling people to fuck off and do their own thing, instead of pissing me off.
If someone is a bully, then i couldnt care less what they think so i say whatever i feel like saying.