A lot of teens go through difficulties, whether it be break-ups, family life, or even just stress. Each person is fighting their own battle and I don't believe it's right for you to add on to their problems. Although some people may smile and act happy around their friends, they may be breaking down on the inside.
So, I believe instead of being rude to a person who may look angry, try being nice or at least civil, because you never really know what they could possibly be going through. By being nicer to others, you have made a difference in both of your lives.