So let's say that there's this jerk. He's always a jerk. Let's say he's a bigot, a white guy that doesn't like blacks, Jews and gays. He sometimes goes out of his way to verbally attack or antagonize them. But he says up front that he feels this way, and therefore decides that everything is fine since he has his disclaimer, as if to say, "You know what you're getting with me by being black, Jewish or gay, so if you feel bad, that's your problem." But is everything fine? Let's examine this. There may be several situations in which it is.

Something bad or very annoying just happened, and you warn everyone around you that you may be a little on edge. Fair enough. These things happen. And it's not like you're always a jerk, like a bigot. However, this doesn't excuse just any behaviour. It only gets you off the hook for being a little snippy.
1. You have good reason not to like a particular person or well-defined group of people. Like people who routinely abuse you, or screw you over. And you have no problem letting them know it. Being mean to such people should be used very sparingly, however. And I don't mean well-defined group as in GERMANS, or people from Victoriaville.
2. You are out of patience because someone is incredibly dense. This is a very, VERY, shaky reason. You had best make absolutely sure that you aren't the one that's stupid. Or that you aren't ever stupid, for that matter. You are rarely as smart as you think you are, or the other person is rarely as stupid as you think they are. This goes quadruple if the other person is in a relationship with you.

So if a person is just a jerk, and that is that, then what reason could excuse them? If a person tells you that they are going to be shitty, and implies that that is all the justification that they need, it doesn't change the fact that they are still shitty people. Are they saying that they aren't shitty because they are being "honest"? Does that trump a crappy attitude? How does this make them less guilty? How does this make them feel less guilty?