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As an alternative, add a faction or trader that basically represents law enforcement to whom you may sell unwanted prisoners for a bounty while avoiding the mood penalty.  They pay you less than slavers, but your bleeding heart colonists don't irrationally go apeshit over the fate of someone that tried to kill them yesterday.  You could also add an option to ransom them back to their old faction, but that might be harder to implement.

Sort of related to Slave/Prisoner Workforce, a faction/trader that represents a law enforcement patrol so you can turn pirate prisoners in for a bounty and avoid the mood penalty for selling them as slaves.  I hate letting guys that tried to murder me go free because they are scum and my bleeding heart colonists go apeshit for selling them or killing them.