>Why doesn't the original queen kill the virgin queens ?
Laying queens don't try to kill anyone.
> I have always read that the first virgin queen to hatch kills the unhatched virgins .
Yes. Virgins try to kill virgins. They don't try to kill laying queens and laying queens don't try to kill anyone.
> I thought the original queen & the swarm left before the virgin queens hatched .
Usually, but not if the old queen is clipped.