Why abduct a colony?

Okay so I've decided to write the scifi story I've had floating around my thoughts for a year or so now. Think Seven Samurai in space. Alien raiders hit a small human colony and make off with the valuables as well as half the colonists. The other half was slaughtered save a handful. One of the survivors, enraged that the human authorities don't have the resources/will to go after them (the colony was a private endeaver; the colonists were warned not to expect protection from Earth), decides to gather some alien mercenaries to go get her friends and family back.

I've got the characters fleshed out, but don't have a clear idea in my mind why exactly so many humans got abducted in the first place. It was the spark for the story (assembling the actual mercenaries) and characterization, but that was about it.

So what would be the motivation:
The shriak, the aliens that hit the colony, are violent xenophobes by nature. Well the violent part at least, as their mental development is linked to the release of hormones triggered by ritualized (now at least) combat. The more I fleshed out the shriak, however, the less sense the abduction made.

One of the mercs is a shriak, who points out that his people don't typically take slaves. They see slavery, which he explains there is not a matching word for in his language, as too merciful a treatment of enemies. On earth, slavery was often the result of conquering an enemy in war. Shriak never did that. They fought wars until one side was completely destroyed or the other side accepted that their way was inferior and adopted the customs of the victor. After that their was no master and slave relationship. "Many voices become one roar" as the merc tries to explain. All shriak share the same overarching culture and language (when most species are far more diverse) precisely because said culture assimilated or flat out destroyed all the other emerging civilizations on their homeworld. As one could guess, this has an impact on their interactions with other species. That and the fact that most other species have intelligent females and raise their biological offspring, but that is a whole other can of worms...

Thus wiping out a colony and looting the corpses makes perfect sense to the shriak character. Even taking a few captives would be understandable. They would be pitted against one another for the amusement of their captors. Taking half a colony doesn't really make sense, though he supposes they could be intending to sell the humans to someone else (the nonchalant way he reasons this to the survivor makes for a memorable scene). Which only shifts the reason for their abduction to another group. Maybe humans make good pets. Or perhaps the shriak want to use the humans to train young warriors. Preferably in a pit. And the human should have a knife to make it sporting.

The other route I was taking would be that the shriak most encounter are similar to the Cossacks of ancient Earth. They are not typical or their people, but are malcontents and exiles that went (or were "encouraged") to the fringe of shriak space to try and carve out a place for themselves. Thus they could just be "mindless beasts" as the merc character views them and might have just taken the humans on a whim. Or they took the colonists in the hopes of nabbing a few skilled engineers to retrofit their gear.

Or someone just paid them to do it.

Any ideas?

Historically slaves are taken for labor, sex, skills, food. Of those, it would seem the first three might be reasons to selectively take only some. Labor you'd take the strongest, skills you would take the miners, (who know how to get a mineral) and then sex, the ones they are most attracted to, perhaps they enjoy how the males scream when they are raped? For food you could assume they would take all of them they could find.

My first thought was that the aliens have some sort of status/prestege attached to the first kill. They have the prisoners for the express purpous of pitting agains their young in unfair combat (unarmed/unarmored and optionally hobbled against well armed and fully armored enemies) as a sort of bloody bar mitzvah.

Maybe there is a retro/fundamentalist cult that rejects the ritualized combat but doesn't have the power to take on the main stream groups so they capture aliens to use as combatants or gladiators.

MASS PROBINGS!!!

I dunno, I just wanted to be a part of this thread.

Food