Any case where the outcome is worth the price. It is difficult to define that further without excluding scenarios.
Intellectually, I have zero pity, sympathy, or concern for those that choose to deliberately harm the innocent, and am not of the opinion that they deserve life. It is however, for good reason, difficult to achieve that kind of distance in terms of emotion. As such, I cannot say how... troubled I would be by such an event afterwards, but it should suffice to state that I absolutely refuse to let others suffer when action is within my ability, and am unlikely to let such get in the way.
Of course, such situations are likely to be... difficult to approach from a reasoned standpoint. Since I have not been in a life or death scenario before, I cannot state for certain how my body and mind would react.