I find the topic of judging someone or groups of people can be a pretty polarizing subject. The argument is usually, on one side, that you can’t judge anyone (for various reasons). But opponents usually argue that you have to judge someone to have an opinion or be insightful about people’s motivations or the reasons they do things.
I don’t think you can ever actually know a particular person. If it’s one person walking down the street or even an old friend, I think it is very hard to ever know someone’s true motivations for anything (a lot of the time I think it’s even hard for them to know their motivations). It is impossible for you to know how much effort they put in to being a better person or fighting their negatively guiding emotions. And since you cannot know their effort, you cannot judge them. Sure, you might not like them or want to hang out with them all Saturday, but it doesn’t mean they’re a bad person.
Conversely, however, judging groups or stereotypes of people can be useful in helping you understand your own values, understand how people’s minds work and debate morality.
So I guess you can’t really ever get mad at a particular person :S