Rahashya summed it up.
It's not what he did, it's how he did. Those three guys had already made a plan to play together after the US olympic stint. There was no need to pull that drama. It was an epic failure on LeBron's PR part, which by the way, consists of his friends not professional. They gave him a bad advice, he admitted it later, but he should've known better. It was nothing but an ego filled display of celebrating self.

I rooted for LeBron since his High School days. Sure I wanted him on my team and am pissed that he isn't there, but the way he did it makes it so much easier to hate him. That, along with the fact that I don't like Heat anyways since Pat Riley went there.