This has always been about principals vs pragmatism.

A person who kills to protect his home and family is not a killer.
Will most likely never become a killer.

But a serial killer and a person defending their loved ones both kill.

The higher problem comes in when you consider that if you do something once, its easier to do it again.
That slippery slope.

However, letting the serial killer rape and murder your family, because you are afraid of that slope is the worst solution.