Marvel kill when it's necessary.
Cap killed when fighting in WWII
But watching winter soldier he just beats the shit out of people mostly. Since they are hydra, and follow a nazi type regime and are shooting innocent people, he will do what it takes to stop them. And usually gives them a chance to lay down their arms.
Same with iron man. It depends on the situation. WHen he's fighting terrorist. A guy shooting in a tank is considered warfare so he's getting a missile up the ass. But when he is in close combat he pretty much subdues peeps not kill. Same with cap.
Widow, hawkeye, thor.