>>877
They were literally designed to be superior to normal humans in every physical respect, and raised in environments that would likely make it pretty hard for them to understand how normal humans think, act or feel.
Space marines have importance and duty. Humans have tiny, feeble and short lives that usually don't accomplish anything.
>>934
They never needed to invent a reason for humans and marines to dislike each other, they've had a damn good one for years.
Space marines on both sides of the faith have invaded Terra at least once. Chaos did it during the Heresy, the Black Templars did it during the Age of Apostasy, and for the most part, the average layman barely has any clue why. No matter who the Marines fight for, the humans know they can and will crush them flat without thinking. An individual Marine is special, important, an investment, serving a higher purpose. An individual human is replaceable, insignificant, an afterthought. And if killing an Emperor-loving human will serve a Marine's purpose, there won't be any hesitation and the humans know that.
As has been said before, barring rare cases, humans and Marines simply can't understand well enough how the other thinks or feels.