It's unfortunate that there are so many abusive tweets/posts directed at him, and that's uncalled for.
However, the media is always going to talk about United players, fairly or unfairly. Being a United player put him in their crosshairs, and being the captain meant that he was always going to be under a microscope. It was up to him to manage the pressure of being the United captain, and unfortunately, he couldn't handle that.
I'm sure he'd turn out to be a decent buy for West Ham, and haunt us by scoring a set-piece header or something, after which the media will claim that we're idiots for selling him.