Well pus is just white blood cells responding to an infection. Without examining it in person the best guess is an old penetrating wound that was trying to heal. How did the buck look/act before the shot?
Depending on the extent of the injury you could be able remove the damaged area and salvage the rest of the carcass or you could be looking at spetic shock which will cause a lack of perfusion in all the skeltal muscle. In other words you will have coyote bait.
Hunt hard, kill clean, waste nothing and offer no apologies.
"In rifle work, group size is of some interest...but it is well to remember that a rifleman does not shoot groups, he shoots shots." Jeff Cooper
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