I printed an anonymous letter about my boss's behavior and now he is completely paranoid

My direct manager has been an absolute nightmare for the past six months, constantly taking credit for everyone else's work, publicly shaming people in Slack channels for tiny mistakes, and treating the team like garbage. Everyone was talking about it behind closed doors, but nobody wanted to say anything to HR because he is close friends with the director. Two weeks ago, after he threw a massive fit over a tiny formatting error in a report I spent all weekend fixing, I snapped. I went to a local library near my house, typed up a brutally honest, two-page letter detailing his toxic behavior, poor leadership, and specific instances where he stole credit from team members. I printed it out on basic paper, put it in a plain white envelope, and slipped it onto his desk late on a Friday after almost everyone had gone home. He found it on Monday morning. Ever since then, the office vibe has been completely wild. He is insanely paranoid. He has been scheduling random 1-on-1s trying to gauge people's loyalty, looking over everyone's shoulders, and watching who talks to whom near the coffee machine. The best part is that he actually toned down his screaming matches because he has no idea who wrote it or who else might have a copy. I just sit at my desk, do my job, and watch the chaos unfold.