Anger is not considered a good moral motivation because it only enablers the person to handle their anger in violent ways. It is not good to relieve anger by acting violently because one's judgment is clouded and one is not thinking rational. Once anger has filled the mind the actions are then unpredictable and the harm can be severer because no one knows the extent of the actions and how it may be carried out because one is anger. Anger is not a good way to relieve stress.
Sentence 1: Violence is considered to be morally good in some instances, so that is not a valid reason for anger to not be considered a good moral motivation.
Sentence 2: You are assuming that when we are angry, we automatically lose our ability to think rationally. That is not the case, for in the event that such a feeling takes over our beings and judgment, it ceases to be anger and transforms into rage.
Last sentence: You might be right, but this is entirely irrelevant debate subject. No one ever said that there exists a necessary relationship between relieving stress and morally good actions.
Even if this is the case one cannot say that anger is considered a good motivation when negative emotions are the driven force behind it. When one thinks of anger it is considered a negative emotion so it cannot be in the same token a good motivation because that would contradict itself. If anger is a good motivation that would seem invalid because someone is always on the receiving end of pain. Whether the intentions are good in the eyes of the person seeking relief it will always cause pain to the other, leading to a never ending cycle of anger. So can pain really be a good motivation?
If one is angered by the wrongdoing that has been cast upon one of their loved ones, it is the love for that individual which causes anger to arise, because if the same thing had happened to a stranger, someone who the angered person did not love, he or she would not be angry in the first place. Anger isn't always driven by negative emotions. In some instances, it can be driven by purely positive emotions, such as love.
Also, you are assuming that only because one is angry, he necessarily has to act upon the anger in ways which cause pain to others. This is not the case. Imagine a time in which you are angry because wrongdoing has been cast upon an innocent party, and you are so angered by the happening that you swear not to ever cause wrongdoing upon anyone who you consider to be innocent. This is one example in which anger can lead to a morally good action.
Sure what your saying could be likely but even in the mist of anger in such a positive emotion like love. The anger that arise in us causes negativity that reflects in some way a negative reaction. What your saying is that anger can be good if it has to do with something like love or a positive emotion. Nonetheless our thoughts cause us to hold in a energy that is unhealthy. If A husband was to get anger at his wife. He thinks of negative things that make the issue of anger relevant. Because their mad at each other that makes the issue worse because each person is harboring negative emotions that can turn into something else...like violence.