The presence of a blood stain isn't what's creepy. It's the fact that not only is the smear like forty feet across, but they're standing in piles of wreckage, bones and pools of what looks like either blood or runoff with iron oxide in it. "What the fuck", indeed. At this rate, I'm not even sure that the guys in the spider tank are safe.
Thinking about this, it's not just the color, but the fact that everything is tilted and off balance. It has the feel that the entire scene is about to come down on their heads. Spotlight effect on the monolith in the center is a great way to highlight the red smear.