This is not a single event/plot point debate. This is the result of the cumulative. I don't think Superman killing Zod would have had nearly as much controversy if he had given even some regard or had some sort of struggle over the crazy destruction and loss of life he was inflicting just by fighting in the movie. It was his total disregard of the loss of human life throughout the film, on top of killing Zod, on top of his passive aggressive (and sneaky) pettiness towards a bullying trucker, on top of his father scolding him for saving a bus full of children, that made people go ask who the fukk is this?
But he was shown all through the film saving people.
Hell, Lois had a wall of newspaper clippings showing all the heroics he had been up to clandestinely in the years between his leaving Smallville and the movie began.
As for his being petty in MoS, I can point to the Donner/Reeves Superman movies for multiple examples of petty Superman.
Like when Supes got his powers back in Superman 2 and decided to go back to that roadside diner to get revenge on the a$$hole who beat him up when he was powerless.
And just the whole giving up his powers stuff in Superman 2 shows a complete disregard for other people. Superman is such a horny selfish b*stard that he will stop being Superman to get in Lois lane's panties. fukk dealing with the 3 evil Kryptonian criminals that are taking over the fukking world while he and Lois bone I guess.
He was a jealous stalker deadbeat dad in Superman Returns who continues to allow a dimwit to take on all the responsibility of raising Superman's b*stard child..
And there are entire websites devoted to how much of a dikk Superman was for decades in the comic books.