There's a difference between dropping your weapons and surrendering or running away though. The threat is already neutralized if they have surrendered, however they are still a potential threat to either you or someone else if they run away.
potential threat =/= immediate threat.
if Whitey Bulger moved in next door, that would be a
potential threat....but that doesnt mean i can shoot him while hes getting his morning paper.
6 guys standing on the sidewalk outside my house wearing ski masks and holding pipes are a
potential threat......however, if they are just standing around not doing anything illegal or threatening, they are not an
immediate threat......and i cant justifiably shoot them and claim SD.
the fact is, a man running away from you is not an immediate threat to your well being....and you cant shoot someone because they
might come back.
the threat is neutralized when they no longer pose an immediate threat to your life.....whether they run away or surrender is up to them....but you cant shoot them because you dont like their answer.
Edit:
hell, if given the choice, ide rather that they DID run away......because even if they surrender, they are still a potential threat, they may get desperate, change their mind about surrendering and try and fight me for my gun, or even pull a second weapon.......if they run away, its just putting more distance between me and the threat.
im not the police, my job isnt to arrest people.....its my job to protect the well being of myself and my family, and ide rather get any source of danger as far away as possible.