I was thinking about 'no capital punishment' in my line of don't be a hypocrite killing people (especially as someone who doesn't believe in an afterlife) and then I remember Kim Jun Un. Perhaps the vacuum that ISIS filled would happen if he was gone... But the guy kills 6 generations of your family for just suspicion of being a little anti dictator. What about the generations not being killed just because?
I consider capital punishment (and punishment in general) as a very flawed concept. In general I see criminal activity as mental disease that is to be therapeutically treated, not prosecuted.
The crime - punishment cycle brings nothing good. As was once stated "The eye-for-an-eye approach will eventually leave all of us blind."
Yet I would use deadly force to prevent (or at least try preventing) an imminent violent crime from happening.