I am watching the 4th season of House Of Cards. When Frank Underwood was shot, and placed in an induced coma, he was hallucinating about a civil war soldier. Who was that soldier and why did he appear?
The soldier symbol appears several times in Season 4 - for instance when Will Conways little son takes Frank's toy soldiers when invited at the White House. Frank bursts out: those are not yours to play with! I think perhaps he takes his soldiers so seriously because the "invation" of the Conways in the WH, also because he is not a father but also soldier stands for fighter in his mind, that he is -behinning with his ancestors- a tenacious fighter, willing to kill in order to win. That might be the reason the soldier appears in his hallucination from the depths of his psyche.
American Civil War symbolism has been deeply ingrained in Frank's character since before season 4. When you think about it, politics is a form of civil war. Frank is fighting his own countrymen, his own party even, for the White House. In season 2, he reaches out to that image when he buries his class ring in the site of his Civil War soldier ancestor's death and it stays with him ever since.
When Frank was Vice President, he visited a civil war reenactment. During that visit, he met with someone playing the role of Augustus Underwood; Frank's great great great grandfather.
Frank is clearly in awe of being connected to his own lineage.
After Frank gets shot, the civil war soldier he sees is Augustus Underwood (more accurately: the civil war reenactor who played Augustus Underwood).
We're getting a glimpse into Frank's subconscious, so I can't give you a definitive answer as to why Frank thinks of him. However, it seems the most likely that it is because Augustus died while fighting a losing battle (since he was a Confederate soldier). If Frank dies because of the gunshot, he considers himself to also have died while fighting a battle. At the very least, he fears that his current plans are a unwinnable and will kill him (politically or biologically).
As it was already mentioned by yourself, Francis was hallucinating about the people he had hurt. Recall S02E12, when Jackie asks about Frank's Civil War battlefield model, he says that it's "Something I've been working on for a very long time. And it's nearly finished. Jackie, help us finish it."
The battlefield model is a metaphor of the Underwoods' road to power, with its planning, casualties, attention to detail. And the reenactor is a personification of all the people that stood in Frank's way.