The blood looks unatural in my opinion, they look like lots of threads, rather than blood. I don't know if this is your personal touch, but I find it strange that you can see the outline of the hands in some parts and none in other parts. I just find it confusing. The way he is "holding" the skull is a bit weak, it looks like it's about to fall. Also the skull looks too small, unless he just killed a miniture alien or something.
Other than that it's very nice. The idea is very cool.