Most of the hate started at Assassin's Creed 3. Assassin's Creed 3 was a buggy game. The hate had started at Brotherhood's ending but Revelations balanced the delicate story that was treading shaky ground. I personally feel that the death of Desmond Miles in AC3 just did it. AC3 was also really annoying in terms of gameplay.
The spinoff games were also really bad. Although Black Flag, Syndicate and Origins are good Unity was a disaster. I don't hate it and underrate it because it is one of the most detailed games I have ever played but the gaming community is really biased so you shouldn't trust them.