He only had to kill one more before they started spilling all of their secrets.
Geth learned about the drug and their process of Taming creatures. It was wrong... but effective.
Their "Captain" would kill random passersby that they found, steal their items, and use the bodies as bait.
They would stuff the bodies with the drugs that their Lieutenant had created, then set them up in a makeshift structure. When the Mana directed the Direwolves to destroy the structure, inadvertently, they would also consume the drug.
While the wolves were stumbling around, their Hobgoblins would beat them senselessly until the Captain could cast an Enslavement spell on them.
This time, the group's instructions were to return with thirty wolves ready to be enslaved. They decided to use Geth's setup to easily capture wolves as they left the Dungeon.
Obviously, Geth and his crew would be killed and stuffed with the drug. He learned a lot from their brief chat.