He's not hated for being a terrible coach, he's actually one of the most successful coaches to this day. So why is he hated? In reality, people hate Belichick for his success.
Belichick spent his first 15 seasons in the NFL as an assistant coach. He then received his first head coaching job with the Cleveland Browns in 1991. He then spent five seasons with the Browns, but ended up with only one winning season. After that, he served as an assistant for the New England Patriots and New York Jets under Bill Parcells.
He was then chosen as head coach of the New England Patriots. Since then, Belichick has coached the Patriots to four Super Bowls. Out of those four Super Bowls, three were victories. He was also named the AP NFL Coach of the Year twice. During the 2007 season, the Patriots became the first team ever to finish the regular season with a 16-0 record.
Bill Belichick has worked under five head coaches and six of Belichick's assistant coaches have gone on to become head coaches in the league.
So why must he be hated? People just need to accept that he could be the best coach to ever enter this league...