I don't hate Halo 3, or any of the other Halo titles, because hate would imply I cared enough about them to get angry. Everything I've seen from that series has provoked a resounding "Meh". No, what bothers me is the ridiculous success this relatively mediocre series achieved - success that has less to do with the merits of the product and more to do with the circumstances of its introduction and medium of presentation - and the impact this has on development of future titles. The idea that Halo is somehow the pinnacle of the FPS genre is troubling, since success and acclaim lead to imitation, and the fact that the term "Halo-clone" has any semblance of meaning illustrates that we already have more than enough would be Halos.
The original Halo is successful because it was THE big fish in a small pond when it was released. If Halo had been a PC FPS originally, nobody would be having this argument, as it doesn't compare favorably on that platform at all - there were/are simply far too many excellent FPS titles on the PC for the well executed mediocrity of the Halo series to make much of an impact amongst the jaded PC FPS fans. Reviewers and gamers would have panned it as yet another over-hyped game that spent way too much time in development only to end up underwhelming. Fortunately for Bungee, Microsoft's purchase of their company and the subsequent shift to an X-Box exclusive release changed their 'so-so' PC/Mac shooter into a 'fantastic' console FPS (which says a lot about the state of the competition at the time).
So yeah, I don't hate Halo 3 or any of the other Halo titles because they aren't bad games - for a game to provoke actual hatred it has to do a lot more than just "not be particularly interesting or exceptional", for that your game pretty much has to be aggressively offensive to the point where the act of playing it is just one continuous insult to the player, and those titles are thankfully quite rare.
The Halo fanbase on the other hand is indeed a collection of aggressively offensive jackasses that engender reactions of intense loathing and antipathy, so I definitely hate them!