Kali had been wondering ever since she saw the meat in the basket. Upon confirming it's a monster's meat.
'What would he do with those poisonous meats?'
At first she thought the slave would be using it as bait but scanning the room again, she found some blackstains on the flooring. It didn't look much but she found another one.
'Blood. He is eating the meat.'
In disguise of helping the slave hooked the uncooked meat and she adjusted the hook and placed it under the fire.
'At least he is cooking it.' While pretending to sleep, she thinks as she smells the meat cooking.
This slave was obviously sold to an aristocrat and since the pit is surrounding the Millano Kingdom and the duchy is placed around this area, it is hard to imagine Ren to purchase slaves just to throw him into The Monster Pit with no gain.