I would tell you that Cthulhu is not the supreme evil, Azathoth is, except that Azathoth is the Idiot God of Destruction, the shuddering mass at the center of the galaxy, the blind, mad, screaming, nuclear chaos at the point where all dimensions fall into one infinitely hot, infinitely dense point of primal, screaming energy where insane huge gods dance and gesture mindlessly to the horrible thin whine of demonic flutes and tambourines. So, since Azathoth really isn't very intelligent, I guess Cthulhu is the supreme evil.