decompose

diːkəmˈpəʊz

verb

(with reference to a dead body or other organic matter) make or become rotten; decay or cause to decay.

the body had begun to decompose

decay,rot,putrefy,go bad,go off,spoil,fester,perish,deteriorate,degrade,break down,break up,moulder,mortify,necrotize,sphacelate,corrupt