I think Dany's a bit of a hypocrite. She didn't have much of a problem with Drogo's brutality or how HE was taking slaves to fund Dany's war for the Iron Throne (yes, she felt bad over the Medicine woman, what about the rest?) but when anyone ELSE does brutal slavery, she's PO'ed over it?
And that Slave Owner was an utter moron to never once think that anyone he sold his slave warriors to might use them against him. Sure, maybe no one ever bought them ALL at once, but you'd think they'd have some kind of failsafe to prevent such a thing.
Do gun store owners give their customers bullets right after they sell them the gun?