Someone once said that you can tell a person's heart—for good or evil—by the things that make him angry. At first, Amy might have judged him innocent based on that evidence. He was angry, yes, but at the torture of the innocent, the threatening of friends.
Now, she's having second thoughts. While he might be angry for the right reasons, his responses are disproportional, wiping out a whole race for the sins of an individual. It reminds her of a dimmer switch, slowly changing from light to dark.