Morally, it's a bit of a gray area. On one hand, it's just fanfiction, a place for wild ideas. But on the other, these characters have a large fan base who see them as positive role models. If the story has a good reason for Tony to kill Steve, like some sort of mind control or a huge misunderstanding that's well - written, it could be an interesting exploration. But if it's just for the sake of being edgy, it's not so great.
It's a complex issue. In the context of fanfiction, it's fictional and can explore different themes. However, from a moral standpoint in the real world, it's disturbing as both are heroic characters. But in the realm of creative writing, it can be a way to explore 'what - if' scenarios, like exploring deep - seated conflicts between the two that could hypothetically lead to such an extreme situation.
Well, it's not really acceptable in the sense that Steve Rogers and Tony Stark are both symbols of good in the Marvel universe. Killing one off in fanfiction seems like a shock - value tactic. But some might argue it's just a creative exploration. Maybe the writer wants to show how extreme a conflict between them could get. Still, it goes against the general perception of these characters as allies or at least respectful opponents.