I thought once they convert, they aren’t supposed to kill them?
This is an age-old trick used by these sadistic monsters. They did it in every era. It all comes down to what they want to do at that moment. In order to secure surrender, they make all kinds of promises they have no intention of keeping.
Another recent/today example. Do you imagine that those 150 Syrian Army guys all surrendered after being told they would be humiliated/stripped, marched barefoot, and then slaughtered?
Hell no. They were promised their lives if the did the correct Shahada prayer and swore allegience etc. Once disarmed and secured, the slaughter begins.
OTOH, if the victors need slave labor, they can keep them alive for that IF THEY WANT TO.
Link above to the full-text Free Republic thread.