I've never understood the hatred. Jealousy doesn't fully explain it. There is more. By all accounts he is a good teammate who is "one of the guys" while others elsewhere are divas. He gives credit, plays hard, and succeeds despite draft status. He should be cheered.
I believe it's a combination of jealousy and nationwide hatred for New England. People can claim it's cheating or whatever, except for that a) they hated Brady long before Spygate, and b) they've forgiven other athletes for far greater offenses. By which I mean offenses that actually occurred and have not been scientifically disproven. But people really hate New England, to the point that they invent reasons to hate people from there. Look at the mental gymnastics people jump through to justify dislike for David Ortiz, the 2016 Celtics, Affleck Damon and Wahlberg, etc. etc. It's a regional thing. People don't like us because they perceive us as being arrogant and thinking we're better than everyone else. And when Raiders/Chargers/Colts/Eagles/Rams/Bills/Dolphins fans realize that we don't even consider their teams as rivals, it kinda plays into that preconception.
That's probably the third element in its own right. A
lot of fanbases, probably north of a dozen, consider the Pats to be their team's greatest rival or something close to it. And in most cases, we probably don't even realize that there's supposed to be a rivalry there. Because their team's most important game in the past decade was some random divisional round stomping that they suffered through, while for us the Pats wrecking some team to get to the AFCCG is an annual occurrence. I bet the Chiefs fanbase is still pissed at how we knocked them out in the divisional round this past year, and meanwhile I have trouble remembering offhand exactly who it was we took out. Jets fans, Dolphins fans, Bills fans, Steelers fans, Colts fans, Ravens fans, Broncos fans, Raiders fans, Texans fans, Chargers fans, Eagles fans, Rams fans, Seahawks fans, Panthers fans, and probably a couple more all consider the Patriots in general and Brady in particular to be one of their most hated teams purely on account of what happens on the field. I know a bunch of Chargers and Raiders fans out in Socal, for example, and they all hate the Patriots at least as much as they hate their actual division rivals. I've witnessed people from both groups rooting for the Broncos--the team that owns their division--over the Patriots. It's irrational to the point of being kinda pathetic in how obsessive it is.