Apparently it's a cycle. When a hero kills a demon lord, the energy-drain curse passes on to them and gradually grows in power. All living beings (stronger people have slightly more resistance) and lesser demons will die in its presence. The previous demon lord was an ex-hero who fell into despair after she was cursed and society rejected her, thus corrupting her into the demon lord.
MC doesn't have this problem for now, since the 4 demon maids are keeping his sanity in check.