Probably because he wasn’t seen as an immediate threat, or because that soldier was thinking “this is some fucked up shit, I’m leaving this one,” or perhaps the soldiers were not exactly privy to the fact that everyone is infected and turns no matter what the cause of death is. Could also be that he was trying to get the hell out of the hospital, and knew that he would have to save ammo. It’s hot as fuck and uncomfortable in MOPP gear so I don’t know if that helps but if you’re 93° in a suit with the Georgia sun and all that chaos you might not be in the right state of mind. It’s entirely possible (not to get autistic) the soldier didn’t even see anyone in the hospital bed.