Victimize

Sorcery

Choose two target creature cards in your graveyard. Sacrifice a creature. If you do, return the chosen cards to the battlefield tapped.

You must choose two targets. You can't cast Victimize targeting only one creature card.
If one of the targeted creature cards is an illegal target (for instance, because it has left your graveyard before Victimize resolves), you'll still sacrifice a creature and put the other card onto the battlefield. If both are illegal targets, Victimize won't resolve. You won't sacrifice a creature.
The creature you sacrifice isn't chosen until Victimize resolves. You can't return the creature you sacrifice because it will still be on the battlefield at the time targets are chosen.
As Victimize resolves, you must sacrifice a creature if able. You can't change your mind and choose not to sacrifice anything.