Think about it this way: good qualities become better the more we have of them. If justice is good, no one would say we should have less of it. If courage is good, no one would want to cut it down. So if anger were truly good, then more anger would be better. But no one wants more anger — because more anger makes things worse. This proves anger isn't good at all. If something gets worse when you increase it, it was never good to begin with. "But anger is useful," someone argues, "because it makes men fight harder." By that logic, getting drunk is also good because it makes people bold and reckless. Some people even fight better when they're drunk. Following this reasoning, you'd have to say madness and insanity are good too, since crazy people are often stronger.
Moreover, qualities which we ought to possess become better and more desirable the more extensive they are: if justice is a good thing, no one will say that it would be better if any part were subtracted from it; if bravery is a good thing, no one would wish it to be in any way curtailed: consequently the greater anger is, the better it is, for who ever objected to a good thing being increased? But it is not expedient that anger should be increased: therefore it is not expedient that it should exist at all, for that which grows bad by increase cannot be a good thing. “Anger is useful,” says our adversary, “because it makes men more ready to fight.” According to that mode of reasoning, then, drunkenness also is a good thing, for it makes men insolent and daring, and many use their weapons better when the worse for liquor: nay, according to that reasoning, also, you may call frenzy and madness essential to strength, because madness often makes men stronger.