In the world of fanfiction, the idea of Iron Man killing Captain America can be quite controversial. Ethically, it goes against the typical heroic and friendly relationship they have in the mainstream Marvel universe. However, fanfiction is a space for exploring different scenarios, so while it may seem unethical to some, it can be a creative exploration for others.
One possible plot reason could be a major ideological difference that escalates out of control. For example, they might have a huge disagreement over the use of a new technology or how to deal with a particular threat, and Iron Man in a fit of rage or desperation takes it too far and kills Captain America.
A third common theme is sacrifice. Fanfictions may depict situations where either Captain America or Iron Man has to sacrifice something for the greater good or for the sake of their relationship. It could be something as simple as sacrificing personal time to train together or as big as sacrificing a chance at personal glory to save the other.
In the comics, Iron Man did not kill Captain America. Their conflicts and battles were part of the storylines, but not to the extent of one killing the other. The relationship between them was complex but not fatal.
In the comics, Iron Man doesn't end up killing Captain America. Their battles and disputes have various outcomes, but not one that involves a fatal blow from Iron Man.
Well, a great Iron Man and Captain America yaoi fanfiction would be the ones that delve deep into their characters. It could be about how they support each other during tough times, like in the battles they face together. Maybe a story where they realize their feelings for each other while on a mission. These fanfictions can add a whole new dimension to their relationship, but again, it's just fan - made interpretations.
The theme of sacrifice also pops up. Their relationship is such that they might be willing to sacrifice themselves or their own interests for the sake of the other. This could be in the face of a powerful enemy or a difficult decision that has to be made for the greater good.
Power and responsibility is also a theme. Both Captain America and Iron Man have different views on how to use their power. Fanfictions often play with these ideas, showing how they might influence each other or find a middle ground. For example, some stories might have them debating about the best way to protect the world.