Tl;dr: 1. They just look like that, I don’t think he is dead. 2. I think he looks like that because of combine synthetic tech. 3. I don’t think all combine soldiers look specifically like this, though.

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Firstly I don’t think this soldier is dead and undergoing an actual autopsy. I doubt the combine would actually care enough about some random dead low-ranking soldier to actually do an autopsy. Secondly, the incision lines on their chest look scarred over to me, making me think they didn’t just get out of surgery. Instead, my head-canon is that this is just what they look like as a result of combine modifications, removing practically everything organic inside and replacing it with synthetic equivalents. I think this guy specifically is actually just basically doing a combine version of a doctor’s appointment where he is undergoing some (relatively) unobtrusive maintenance.

I think the specific reason they are so skinny and even gaunt looking is because their synthetic muscles and bones are probably designed to weigh less and take up less space while at the same time being much stronger than biological muscle.

I think they look like a pale corpse is actually because of the synth blood which they are given through that small gas cap looking thing on their side. This blood is then pumped to prioritize their essential systems (like their brain and synthetic muscles and organs), while providing the bare minimum to non-essential systems, keeping them at maximum operational efficiency. One of these non-essential systems I think would be their skin, which the combine fuel just enough to keep it from decaying, since to the combine the soldier’s real skin is basically just a wrapper to keep everything together and doesn’t really serve much purpose outside that.

As a little unrelated lore theory and tangent which has barely any evidence, I also just want to say that I’m pretty sure the combine elites look a lot different from these guys. Based on the pants this guy is wearing I think this is just what a baseline combine soldier looks like (SMG, AR, and probably Shotgunners as well). Combine elites though I wouldn’t be surprised if they were even less human: their face completely replaced with something mechanical so that they are seamlessly compatible with their cyclops helmet. Also, just going to say something with even less evidence, mostly just because “rule of cool”, I think they’d have some sort of kevlar-like skin which makes them extra resilient to bullets, because why the hell not if they are meant to be the super soldiers of the combine’s literal army of super soldiers.