Explanation (spoilers from the webcomic): In the original webcomic, the story has progressed to the point where we've learned that Zombie Man's special power is the ability to heal and regenerate himself, even from the most extreme of injuries, and even if his body has been mostly destroyed. (Which explains how he can disembowel himself just for the purpose of doing a joke.)We also find out that he was one of the earlier creations of Dr. Genus and ever since escaping from the House of Evolution he's been hunting their organization down to try to eradicate it. Eventually he finds Dr. Genus and Armored Gorilla, the only two remaining survivors from after Genos and Saitama destroyed their organization early on in the series, and is shocked to see that they have given up their evil ways and are running a small business together.