I actually really like the main character! Hes like one of the few Male protaganoists that finally has a pair! He also doesn't fall into the like 5 overused cliches in all of anime.
I guess though with the killing people as a kid thing some folks would be turned off, only if they dont realize that HE realizes that everyone is living in fear and living in the birdcage of a City, and that his restless frustration of constantly living in fear of the giants (however small that might have been at the time) and the combination of seeing humans do that to someone, and the situation he found mikasa in (he did see mikasa's mutilated parents bodies and figured the same would happen to her if he didnt act)
His actions there make perfect sense to me.
But I guess you may think that he is suffering from bit mouth main character syndrome Where a character is all talk.
Personally I find him to be a bit of fresh air in comparison to other protagonists out in the manga anime world. he isnt retarded for one(the steins gate lab coat guy takes this one)
knows what he needs to get done…and does it
AND is not perfect omfg I am so gAr and has room for growth. . .
AND the flashbacks showed that he HAS grown, and how other characters contributed to his development in those flashbacks
oh and I think the giants are just some war machine gone bad, Probably from a war 100 years back and the giants were just some "Secret" weapon from another continent that was unleashed on another populace(that we are looking at now) and were never "Turned off" Hell it could be that the other nation still knows about it and keeps upgrading the giants and sending them to kill(hence the uber titan that just 'Happened' to be tall enough to breach the gate 100 years later)
I mean its already been hinted that the giants can be . . .'Driven' you know the nape of the neck being about human size+the fact that the MC can ride it, and the Doctor knows a LOT and then disappears etc etc. . .points to something more sinister and/complex than just 'Mother nature getting revenge' and since they apparently can survive for 100 years without getting food(humans)