User blog comment:Prime ShockWaveTX/Your Top 10 Most Vile Villains from Villains Wiki/@comment-4851935-20150120125638/@comment-2059440-20150126022643

in  strict practice of Christian theology the Devil can not be a Complete Monster for the reason that God allows Satan to tempt mankind : many faiths believe Satan is still either in service of God (as an avenging angel type) or is allowed to oppose mankind until Judgement Day as part of a deal he made with God during his fall.

This is due to the belief that God is truly omnipotent and thus no evil could exist without God allowing it.

for a Complete Monster version of Satan to exist one would have to (at bare minimum) have a dualistic approach in which God and Satan are equal in power (which is considered heretical in mainstream theology but is accepted by some other sects).

In fiction most writers tend towards a much more dualistic approach, even then it is rare for a true "equal footing" scenario, meaning Satan is usually an agent of God or an opponent that God allows to exist (and thus just narrowly avoids CM status as a "necessary evil").