This discussion is pointless because it's just a proxy war. Everyone's position is completely determined by what they think of Bannon (or Trump really), so no information is exchanged. Here's a thought anyway though. The image he used (heads on pikes) is gruesome and is also a common metaphor for doing something dramatic to deter unwanted behavior, also known as pour encourager les autres. In my mind that usage tilts the remark away from "let's literally put their heads on pikes" to the more metaphoric. Now try this thought experiment: come up with some new grisly image of a way to murder someone, without any history of being used as a metaphor, and imagine Bannon had used that image instead. That would tilt the remark more towards being an actual threat.

Other people in this thread have made a similar better point. If he had said "heads should roll", that's not a call for decapitation. Another obvious point is that he's being an edgelord, going too far but not exactly all the way, in order to get attention. Edgy references to civil war are the same. That's not starting a war, but it's not totally not starting a war either. There aren't precise lines to draw here, probably on purpose.