The logic behind this is that taking one life to save others is "permissible", legally, and ethically by police negotiators, SWAT teams, etc.; but from a moral stand point, that is a more individual take on the situation. Morally, a life is a scared possession that no one has the right to take other than God, but even a Christian in this moral situation would say that the Bible says, an eye for an eye; yet to argue even that one could say that unless the hostage taker has actually taken a life, his should not be taken. To split the difference, the negotiator could suggest injuring the hostage taker rather than killing him and allowing officers to storm the premises and seize him immediately following the shot for injury to keep him from retaliating in shotting a hostage, but then that is a decision that may not be an option depending on the situation. Telling someone to kill another is a hard situation to handle emotionally, psychologically, and for all purposes morally and mentally for there is always the "What if?" question that will forever haunt you. What if the gun wasn't loaded? What if the man was simply off his medication? What if he merely wanted to talk to someone and get help? What were his true intentions? Was he capable of killing anyone? What if???
The moral issue for the people involved in these situations is normally decided prior to ever taking this type of position, they have to look at it as a job and do that job to the best of their ability. However, the human factor changes every situation. A hostage taker that has no compassion for the hostages, has injured or already killed them, would be easier for one to feel justified in having lured to a window to be shot where a family man that just lost his job, his house, and his wife wants a divorce who is holding his family hostage so that he has one last inch of control over his life would be more difficult when you can assume that he just needs someone to talk to. You sometimes have only a split second to make that decision, but in my "opinion", I would say that each situation must be handled individually yet still adhere to protocol. Ask the same questions, follow the same plan of executing a compromise for hostage release, and gaining the trust of the hostage taker.......use all the information to make a call of whether taking this person's life would be the ONLY just way to end a bad situation or if injurying that person would give the same results. Could you sleep at night knowing you took that life, do you feel totally justified in the decision you made, would you make the same decision if asked tomorrow? Perhaps the answer may always be the same for someone who sees this as only a job with no moral implications, but for those that do believe in and answer to a higher power, perhaps that is the power you should let guide you.