Baron Zemo is a major villain. He often schemes and creates chaos that forces Captain America and Iron Man to deal with his evil plans.
There are also villains like Ultron. Ultron's threat to humanity forces both heroes to put aside their differences at times and team up to combat this mechanical menace.
The Mandarin is a very prominent villain. He has ten rings with different powers which pose a great threat to Iron Man. Another one is Obadiah Stane, who was Iron Man's business rival and later became the Iron Monger. He was a formidable foe in the early storylines.
One interesting storyline could be their ideological clashes. Captain America and Iron Man have different views on things like government control and superhero registration, which leads to intense conflicts within the story.
Sat - Yr - 9 is one of the main villains. He is a powerful and devious adversary who often challenges Captain Britain's beliefs and abilities.
Captain America, Steve Rogers, is the central figure. Peggy Carter also plays a significant role. She is an important ally and has a special relationship with Steve. Then there are the Howling Commandos. They are a group that works with Captain America in his battles. There's also Nick Fury, who sometimes appears and has his own influence on the story.
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.
Yes, in some storylines, Iron Man has come out on top in their battles.
Yes, Iron Man 1 did have the image of Captain America's shield.
Aldrich Killian is another villain. He creates the Extremis virus and uses it for his own malicious purposes. He tries to get revenge on Tony Stark and cause chaos with his new - found power.
There's also Justin Hammer. He's a rival to Tony Stark in the business world. He's jealous of Stark's success and teams up with other villains to try and bring him down. He's not as physically powerful as the others but is quite devious in his plots against Iron Man.