For all those questioning it, the guy at the end is the general that trained him in the other world and he formed a very close bond with, akin to grandfather and grandson.
Being said, the demons really are about to seriously wipe themselves out. If reason stands, chances are that he was holding back when he beat the demon lord and the inly reason his grade A crash out against the generals wasn't a total wipe for them is because his body is still acclimating to his power.
These idiots are literally playing with something that has eminence in shadow levels of nuking potential. If they zombified his waifu then nothing in any world will save them.