Question about Tywin did to prisoners at Harrenhal (season 2, he was there with Arya)

what exacly happened? tywins people were torturing prisoners for information, then tywin came, and tell them "we dont have enought man, dont kill them, make them do labour instead".

when rob stark came here with his mother, all of the workers there have been killed.

so, what exacly happen? Did Tywin give the order to kill them all? When there was the "assasination attemp", is it when all the killing started? If yes, what were the reason to kill them, and were there any alternatives? like enslaving them on different place?