People are jealous of his power, and become angry when they see him "curse" his powers. It's the same kind of contempt people have for wealthy people who are unhappy.
Superhero movies are "supposed" to a power fantasy, audiences want to self-insert themselves into a tall, confident, attractive person and see that person succeed. That person can be a smug asshole as long as they're a winner. Superman should always be winning, and always be smiling, because one should want to be Superman.
Zack pbuh wanted to portray Superman realistically, understanding that possessing so much power in real life would not necessarily be "fun" and would actually subject you to a great deal of responsibility, angst and controversy. The same applies to the depiction of death.
In Iron Man, a man attempts to shoot Iron Man but the bullet ricochets off the armor into his face, killing himself. This is played for laughs. The idea of being so bulletproof that anyone who would try to shoot you will die instantly is a pleasurable fantasy.
But when Superman must kill Zod, he resists the act. He does not want to kill, he pleads with Zod but is ultimately forced to let him commit suicide by cop. He does not blast Zod with a colorful laser, he does not drop him into an explosion, he breaks his neck with his bare hands and we see the corpse fall to the floor.
This is not power fantasy, this scene begs the audience to consider what it means to take one life, when most other films in the genre will have the "hero" now down horses of faceless badguys and follow-up with a one-liner to show unaffected they are with killing.
The way Wonder Woman gets away with killing people is by not showing the humanity of the people she kills, nor lingering on the death itself. We don't actually get visual confirmation that any of those men die at her hand, it can only be assumed because the movie's moves on. The film also explicitly condemns war and killing in dialogue.
