It´s bad from a Bullsfan perspective, moreso for someone who don´t like the Heat if they were to win.
There is a lot of reasons. I can´t remember where i saw someone write this, but it shows very well why so many don´t like him:
"what Lebron James did is like dumping your girlfriend on the jumbotron. Not only does your GF hate you, but everyone else is like "Dude, wtf.""
The Decision, the tweets, doing what is "best for your family", loyalty, not one not two not three not four not five not six not seven, you'll all go back to your crappy lives tomorrow, I spoil people with my greatness, I just want to chill, don't dive for loose balls near me I might get hurt etc etc.
Lebron is probably the best player out there right now, but he is impossible to root for. At least for me.