So... This really depends on age... so this is a rough theory. But...
So if you notice, everyone that died because of a bender died of FIRE bending. So what if Amon is a firebender (which could explain his superior agility, much like Mako's 'Cool Under Fire' technique) who kills people JUST SO that their loved ones may hate benders? To help him? He could be pretty old (doesn't mean that he's not a bamf. Look at Tenzin. You saw how he fought in Ep. 7) and killed Bolin and Mako's parents as a young man, later taking Asami's mother.
There are several things wrong with this theory, but there are also arguements.
1) Why would he hate benders so much if he's one himself? Maybe he doesn't. Maybe he just wants to start a war out of his own problems that don't involve bending.
2) If he's the bender behind all those deaths, why would he take his parents' lives? Maybe he didn't, and they dies of some other cause, or, like what happened with Aang when he burned Katara, Amon couldn't control his bending and took his parents' lives accidentally. This could conflict with 1, being as so: Maybe he does hate benders because he's so upset with himself for what he did.
3) He can't be lying. The Lion Turtle said that in order to take one's bending they have to be pure. Not necessarily good pure, but pure. That would mean that he has to be telling the truth. Yeah, matbe the spirits did give him that ability, but then again, on another blog of mine, what if he's just making it seem like their bending's gone?
Still, this is a rough theory, but hey, whatevs.