Morality is to determine what is right or wrong from an individual perspective. I think it is never right to kill a person. At the same time I think it can be justified because justice is more related to external factors. Therefore, killing someone who causes death on others and will not stop until they are stopped is not right, but just. Then you do introduce a new killer, who should still receive some form of punishment. I realise the big problem with my approach is "who gets to call the shots with said judgement and thus what I say is viable only theoretically.