I have the right to defend myself and other people. In certain circumstances that might mean injury or death to another person, but my intention would be defensive, with the injury and death an unwanted side-effect. At all times I should use the minimum possible force necessary.

That's the principle anyway. And of course there is room for disagreement about what is the minimum necessary force. That ranges from people who would claim that it is justifiable to kill as a way of deterring other people to those who would say that avoiding killimng or injury at the cost of being injured or killed is the only way in the longer run to stop the cycle of violence.

But the same basic principle of minimum necessary violence should be common ground - and that includes a recognition that in all circumstances killing simply for vengeance is wrong.

That's the principle anyway. Human beings being fallible, we don't stick to that, and that is very understandable. When we go beyond what is necessary we may be entitled to be forgiven - but we should never try to say that what we did is right.