On the internet, I see Christians praising Garrett Swasey, the policeman who was shot and killed by Robert Dear. In one respect, that makes sense. People who hate Christians are blaming the attack on Christian extremism–although, from what I've read, there's no evidence that Robert Dear was theologically motivated. So the counter is that a prolife Christian (full-time policeman and volunteer copastor) died attempting to save the lives of others from the crazed gunman.
That's a good counter in the sense that it answers the critics on their own terms. It does, however, raise ethical questions. I suppose a policeman has a professional duty to go wherever the dispatcher tells him to go.
However, this is the larger issue: Is there a moral duty to intervene to save the life of a killer? Suppose Pablo Escobar is wheeled into the ER with a pulmonary embolism. Do the physicians have a moral obligation to save his life? You see, by saving his life, they ensure that he will kill even more innocent people. You patch him up on Friday and he goes back to ordering hits on Monday.
Refusing medical intervention in that case isn't the same as killing him. The doctor didn't cause his pulmonary embolism. The doctor didn't inject him with potassium chloride. The doctor simply let nature take its course.
Sometimes letting person die is equivalent to killing him, and sometimes not. That depends on the circumstances.
But there's no moral obligation to save the life of a contract killer. People in the business of taking innocent lives should kill at their own risk. They are not entitled to protection. You can't obligate others to rescue you in that situation.