But space marines can be complex. Read some of the Ultra Marine novels. They have to deal with social interaction with non-marines to coordinate events, especially protecting their area of space they govern. Then you have the ever threatening web of chaos, as all Space Marines know the only thing that separates them from being a CSM is their faith in the emperor.
So I think doubt and fear are parts of being a SM, not the fear of that ork or nid over there, but the fear of failure, fear of corruption that drives them to do thier best beyond just their natural aptitude.
Besides in Role Playing Games you play the exception to the norm. The character that is different from his normal stock and brood. So PC Space Marines would be more open minded, perhaps on the edge of rebellion or being excommunicated. You would not be playing your standard battle brother, but someone who has more destiny and fate about him. A hero among heroes, or at least a featured character in a story instead of just another power armoured garbed space thug.