"...do not become anger..."

The following comes from Purification of the Heart by Hamza Yusuf (Starlatch Press, Bolingbrook, IL., 2004). The bookstore has a great selection of Islamic books, children's materials, CD/DVDs, and other Islamic media, just as they say. Everyone will gain from something there.

According to scholars, like Imam al-Nawawi and others, when the Messenger of God (PBUH) said, "Do not become angry," he meant do not allow anger to lord over oneself and cause the loss of one's comportment. In other words, do not become anger, its embodiment, such that people only see your rage. Instead, control ager and never lose control. Scholars have likened anger to a hunting dog: without training, it will never retrieve what its owner needs nor will it point a person in the right direction. So anger is something that needs to be trained, not abolished, for if people completely suppressed their sense of anger, many of the injustices of the world would not have been opposed and tyranny would have gone unchecked. Without anger, people would go around with complete impunity and commit heinous acts without resistance from the people. Corruption would cover the face of the earth.

And also see this and the brief and comprehensive commentary give here:  

Hadith 16. Anger



On the authority of Abu Hurayrah (may Allah be pleased with him): A man came to the Prophet (peace and blessings of Allah be upon him) and said, “Advise me” He [the Prophet (peace and blessings of Allah be upon him) said, “Do not become angry” The man repeated [his request] several times and he [the Prophet (peace and blessings of Allah be upon him) said, “Do not become angry”.

Reported in Bukhari