People are trying to sterilize rats or give them birth control now vs kill them. If you kill a rat, you still have the environment that could support a rat and one is liable to more in from the populations nearby you didn't kill. If you sterilize the rat and release it however, it keeps other new rats from moving in potentially in the maintenance of its territory and it could be doing this job for you for years while you try and sterilize the rest of the area. And eventually these rats will die, and if the great bulk has been sterilized in some way then you may actually eliminate or push back sterile rat populations out of certain areas.