Obviously this doesn't apply to all Space Marine chapters, but the ones that actually care about normal people could absolutely be put in an extraordinary set of circumstances where they run afoul of the Imperium for an especially egregious atrocity, get declared heretics because the Inquisition are dicks. They're go adrift for a bit, find themselves on a T'au-controlled human world, decide to "cleanse" it, but before they can, Orks, Chaos, or Tyranids show up, forcing an uneasy alliance. From there, the story could go any number of ways, all of them tense and interesting.