IMO the biggest problem about this manga is just that there’s too many assholes created by the Goddess of Assholes. That’s why everything felt sucky in general; we’re surrounded by assholes.
I mean, assholes attract other assholes. And it would only make sense that the patrons Goddess of assholes is also an asshole.
And I do give some leeway for understanding why the summoned heros are assholes. They're in their late teens, given tremendous power, kidnapped from their home and families, are trapped in another world with only one "promised" method of returning home, are, for the most part, left completely unsupervised, and the moment they arrived, they got to witness first hand what happens to those who are not deemed strong enough to please the Goddess.
Maybe not everyone of them, but certainly many are likely experiencing a tremendous amount of anxiety, fear, frustration, and grief, without a therapist present to help sort out these puberty hormone boosted negative emotions. They just went from a state of pure helplessness when they were summoned, to immediately being given absolute power above that of just about everyone else in that world. Their minds would obviously latch onto that power and will do everything they can to never let go of it.
And what teenager isn't already kind if an asshole even without the excessively lethal powers? And once given these powers, they don't believe they should have to listen to anyone weaker than them tell them how to live their lives.
Even the MC is an asshole, but he was already different than the others going into all this. He has had to deal with shit his whole life, and has been the one on the receiving end of said shit for most of it. So he has kind of already gone through everything everyone else is currently going through, just without the powers.