Soulless humans?

[Schillaci]

I think the point was that if there is nothing there then Evil can't be there.
If you are devoid of everything then anything you do is based on instinct and instinct alone can be neither good or evil.

I would also question that a Soul is not the Good/Evil decider for the very simple fact that the 'Soul' is something that apparently every human has, yet we know that there are some Humans that are evil even in this world.

Potential for evil. Evil is not a state of being, it's a matter of perception.

Also, the question of the Demon Soul.
I have not heard any Demon say they are soulless except the Vampire, however if we assume that all Demons are then why are some Demons good?

Demons like Lorne are impartial. That he helped Angel, has no reflection on his allignment to a force of good or evil. He puts those on their path, whether their path is 'evil' or 'good'.