It's o,k. to get angry. A matter of fact, it's o.k. to get really mad. The key, is what we choose to do with our anger, how we express it and how we've learned to react to it. Our expression of anger can be productive or destructive; it can be respectful or disrespectful. If sometimes we over react and blow a fuse or bust a gasket, and express our anger in a way that we don't feel good about and that doesn't feel appropriate to us in retrospect, we can say we are truly sorry. And, that can be a mutually beneficial learning experience. (Caveat - If yelling or screaming becomes a routine pattern, your apologies are meaningless, and violence is never acceptable, no matter how mad someone may "make" you.)
The honest expression of anger feels good. It's energy, it's release, it's expression, it's speaking up. It's communicating a strong and important message. It's when anger gets pent up, unexpressed and righteous that it no longer feels good to express. Or, when we feel we have to wait until we are undeniably justified in our anger to allow ourselves to express, it is not a satisfying experience.
The other day, a neighbor of mine lost his cool. We live in dog country, and he was fed up with the local hounds pooping on his lawn. He had been frustated for some time. He had long and clearly communicated his stance, in person and with posted signage, and he'd had enough. He wasn't being heard, and he wasn't being respected, and he was ticked, "Get your@#$% dog off my lawn," he shouted at an unsuspecting dog walker. Afterwards, he felt ashamed.
Lots of people are not comfortable with their own anger or anyone else's. We don't know how to express it or respond. We feel fear and shame on both sides of the issue, whether we are the angered or the recipient of someone else's emotion. But authentically expressed anger informs us and others where our boundaries are and when they have been crossed. This is super important information and it is valuable for us all to appreciate what happens when personal boundaries are not respected.