I agree that the guy wasn't very smart to tweet something like that, but the big problem here is that Mike can't act like a grown up and ignore this or something. Instead, he chooses to maximize the problem by calling him out on his social media and accusing him of deleting the tweet, calling him a coward. Grow up, Mike, just ignore the haters/trolls.
Imagine what would happen if James reacted like that every time someone posts on his social media that he can't sing anymore or that DT should get a new singer. Imagine if Mangini reacted like that every time people call him a "robot with no soul" or something similar. But they don't. No one here is saying that what this guy posted is correct, but MP really needs to calm down and think things through before replying to something like this.