Of course I tried talking to him. When he made those changes without telling the rest of the team I asked him in email and on Slack about them. He never answered.

I did exactly as you say. I never go straight to managers without trying to settle things with the principals first because that's chickenshit. He wasn't interested.

All he cared about was clearing his task list, which he put on a big show of doing; he was the only one on the team who sent out emails about his commits, even if they were totally inconsequential one.

The guy was a jerk.