I've meditated on this matter a bit, having had to come to peace with some form of violence during my time in the armed services.
It comes down to this; Do unto others as you would have them do unto you. You can kill someone if you would truly wish someone to kill if you were you in their place. There are some actions that I would wish to be killed for in order to prevent me from doing them or repeating them. If you can genuinely appreciate a rebuke of this severity then you can kill in good conscience.
Although simple in principle, meditating on this will reveal this principle is extraordinarily keen. It's application must be precise.
It comes down to this; Do unto others as you would have them do unto you. You can kill someone if you would truly wish someone to kill if you were you in their place. There are some actions that I would wish to be killed for in order to prevent me from doing them or repeating them. If you can genuinely appreciate a rebuke of this severity then you can kill in good conscience.
Although simple in principle, meditating on this will reveal this principle is extraordinarily keen. It's application must be precise.