I've kind've lost the context of this argument in particular.
I'm in favor of using lethal means to defend your home and property from people who wish to do harm. That doesn't cover holding the bad guy captive for a few days before executing him, torture, chasing them down the street, ect.So, just so we are clear for everyone here; you are in favor of chasing people fleeing from your premises and killing them AS LONG as you kill them before they get off your property? Good to know.
Edit: Also, not chasing someone is not the same as retreating, since you brought up english.
Legally, if you wanted to shoot the crook who tried to rob your house/rape your daughter/whatever as he was running across the lawn, you should be in the clear as the bad guy is still on your property and isn't necessarily "fleeing the scene".
You shouldn't be forced to hide or "retreat" into a corner of your home, you should be able to defend it from the front porch. If you wish to make the decision to retreat into a corner of your bedroom, do so, however don't make a law that forces people to trapped in a corner before using lethal force.