Why can't we just put a wall around the state of New Jersey and throw the convicts in there? Technically a prison, but we don't need to pay all the upkeep of a prison, beyond making sure the wall isn't breached. The land is decent enough that they can grow food. Its not killing, and it removes them from society. And it fixes the problem of New Jersey existing. That shithole is an embarrassment and should be put to better use.

My point is, all the reasons for killing are moot while there are other options. 

The reasons you mention for killing sometimes being justified are really only two scenarios. Self defense and rebellion against oppression. And this is why IMO we should stop violating the 2nd Amendment. Those two scenarios don't carry over to the justice system. Justice is only administered with the defendant in custody, which means they don't pose a threat. The threat is neutralized. Justice can't be revenge.