Slaves that behave, meaning subservient while being used, completing chores etc. May be given a choice in their death. Those who serve should be rewarded after all. Being said, most choose crucifixion; an effort to try what they've always craved. Others realize that they want to limit their pain, given the torment they'll have faced thus far.
But as stated, behaving slaves will be given a choice in death and in certain ones such as crucifixion, they may choose between a more sensual crucifixion; as to be more of a crowd pleaser. For the sadist/masochists types, they may choose a more painful crucifixion with added torments. Extremists slaves that may behave can also opt for a more violent death.
Slaves that do not behave during their confinement will be executed in whatever manner of my choosing, typically a non public and unceremonial gunshot after violent torture.
Slaves that are substandard or that do not impress will not be granted special treatment in any way.
Again, slaves may volunteer for their execution at any time, as these will typically happen daily. Just know the longer you wait to volunteer, the more punishment and agony you'll endure in the dungeons.