If the target creature is an illegal target by the time Torment of Venom resolves, the entire spell doesn’t resolve. The creature’s controller won’t be tormented.

Because state-based actions aren’t performed while Torment of Venom is resolving, the target creature is still on the battlefield while its controller is tormented. If that creature has an Aura attached to it, that Aura can be sacrificed.

While resolving Torment of Venom, the creature’s controller chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Then that player discards that card, sacrifices that permanent, or loses 3 life. That player can always choose to lose 3 life, even if they have cards to discard or nonland permanents to sacrifice.