The bandits discussed keeping him alive for his ability as if he had joined their group. But he was being held against his will and would likely flee first chance he gets. And he now knows where their hideout is. This was very high risk with potential for high rewards assuming they could keep him from running and complacent enough to do what they want. If he ever ran they lose all their loot and have to relocate. Or kill him and just lose all their loot. The least risky thing to do would be to either loot everything and send him off when they first met him.
"He has storage? GREAT, we hit the jackpot; more loot! Leave it all there, and your clothes, and turn back. Maybe you'll die? Not our problem. WE let you live, the rest is up to you."
Or capture him, and sell him to a slave merchant.