The soul is energy. It is renewed by doing good, loving, listening to your conscience, etc., and it is depleted by evil, hate, revenge, etc. It is the sole (pardon the pun) criterion by which you are judged - heaven or hell. If you have no soul (eg: a vampire) you can't do "good" deeds. They have no meaning, and your conscience is stuck on "evil". Any "good" deed done will only be an unintended byproduct. Any love, duty, etc., felt is meaningless, a mockery of the acts that renew a soul.
Anyone with a soul can do good or evil...they always have that ability, but as you do more evil, it gets harder to do good (see above).