that is gnarly!!!!
Looks like Trauma - very sharp to boot. It actually is in a good spot for the buck in terms of the peritoneal cavity is not exposed, etc. I am guessing some sort of infection will set in, but there is a chance with it being all muscle injury he could survive.
Muddy probably remembers this from my time in Iowa but I killed a buck that had easy a 12" gash similar to that one that was also fairly deep (you could see the spine). I am assuming the neighbor hit that one within 24hrs of when I shot him. He was running does all over when I killed him, didn't bother him one bit.