Honestly, with that numbers if Kanki actually let them go they would came back and Qin would suffer even more. That being said they should have sold them to slavery instead. That way the hatred would be redirected to other side.
no resources or time to handle all those prisoners. returning them to zhao is just nonsense, so there was no other way. they only have to blame themselves for surrendering
Yeah, suprisingly the execution system was actually pretty nice. It was only at the end that it became awful and that was because the executioners got lazy.