But I interpreted your response as essentially saying "Anger-at-injustice really does lead to fairly directly evil." Your example does not support that assertion.

It certainly does. In reaction to one evil, Naziism, Germans could go and support a second evil, Communism, which to judge by its global body counts, was many times worse than Naziism, which is exactly the sort of reaction Brin is ridiculing: "oh, how ridiculous, how could getting angry at evil make you evil too?" Well, it could make you support another evil, perhaps even aware of the evil on the theory of 'the enemy of my enemy is my friend'...

I don't know how you could get a better example of 'fighting fire with fire' than that or 'when fighting monsters, beware lest you become one'.