Sometimes, when you are dating, you see something that you think might be a deal breaker. I feel that it is best to take a wait and see approach when this happens. If the behavior is repeated and is really intolerable, or if the person is mean to someone who is unimportant to them, then.....NEXT. But if you find that the incident is just an aberration, then not being quick to judge might get you to a better, deeper relationship.
Alain de Bottom claims we will all marry the wrong person, but it doesn't matter, because:
"The person who is best suited to us is not the person who shares our every taste (he or she doesn’t exist), but the person who can negotiate differences in taste intelligently — the person who is good at disagreement. Rather than some notional idea of perfect complementarity, it is the capacity to tolerate differences with generosity that is the true marker of the “not overly wrong” person. Compatibility is an achievement of love; it must not be its precondition."
My parents have been married to each other for all of their adult lives. They don't always agree, they sometimes frustrate each other, they don't even seem happy most of the time, but what I see in both of them is the capacity to be generous to each other, and to support the other one always.