Hmmm. In Christianity, (and most other religions, if you don't like Christianity) suicide is a sin, and there's no exception if you've committed a terrible crime. How would him have deliberately killing himself atoned?
Well to perhaps oversimplify, there are two schools of Christian thought, the permanent salvation school and the tripwire school.
If the permanent salvation school is right, a salvation could not be lost, although a reward could be reduced, by exiting the world via suicide.
But anyhow, he was a monster, whether ultimately saved or not, at least he WAS a monster. We would like to pride ourselves that we would never be monsters. But we can’t. We are all sinners doomed to an eternity as Satan’s slave in hell without salvation. Just how far our downfall might go before salvation began — is not in our hands.