I'm using "right" in the sense that a programmer speaks of "access rights". An access right is the ability to do something, not moral approval. Rights in the sense I'm speaking of here simply refers to making a set of actions reachable in the brain's planning trees, if that makes sense.

I've found, though, that asserting that one has the right to do something is helpful in imperatively making this connection in the brain, so that's the word I use. (It seems in many people to elicit an accompanying "territorial" emotional response, that may or may not be related to the mechanism used to mark actions accessible or inaccessible in the first place.)

A difficult question for me to answer. It comes tied up with other realisations and beliefs:

• There is no God, no rules for Right and Wrong written down in the fundamental nature of reality, no external standard. I need not bend my beliefs of what it it right and wrong to do and, more importantly in this instance, feel to anyone else.
• There is no 'fair', no 'justice', except to the extent that I or other people who share similar preferences make it so. Other people may get away with insisting that what they are doing is Right, with the implication that you do not have grounds to be angry. They may be able to socially enforce the suppression of anger at their actions with clever reframing or outright force. But you need never subject your own feelings to their demands. You don't need permission to be angry.
• I have anger for a reason. It's there to tell me when the outside world has hurt me in some way. When we choose to suppress anger it can deny us knowledge of what we want or need in a situation. For all but the most self aware individuals emotional instincts know more about what they really want than conscious beliefs.
• Letting my anger be my own, rather than trying to insist it match an external tribal consensus frees me from attachment to things I can't control. Other people can be F@#&$ if they want to, and I can be angry about it if it serves my purposes. Sometimes it does.
• Anger makes me think better. I am more focussed, extremely strategic and much harder to manipulate. It isn't a long term option but in the short term anger is damn helpful for me. It gets me out of bad situations and opens my eyes to all sorts of opportunities that I may otherwise have been too nice to acknowledge. (Contempt, on the other hand usually just gets me into trouble!)
• Once I start allowing my anger to work with me rather than fighting it it doesn't have a destructive influence on me. It is like a trusted military advisor that cooperates me. I don't always follow its suggestions but sometimes I do. Once the instincts that can be considered the 'angry part' of me are properly integrated with the rest of me they come to trust that the rest of the brain will cooperate to meet its goals. It will not then be tempted to sabotage the goals of the rest of me.

I hope this answers your question at least partly.