Sometimes emergencies arise requiring that somebody gets whacked. Your sudden hostage-taking is a pretty good example. Terrorist grabs a small child, holds a knife to its throat, and says to whom it may concern that unless X dollars are forthcoming or X somebody is released from jail, the kid gets it. This situation calls for an immediate bullet in the terrorist's brain. End of story.
Wars are sometimes situations that present continuing or continuous emergencies, often in multiples.
Whole nations, races, religions can be lost to wars, and sometimes the good guys have to kill enough of the bad guys to get them to quit. In a sane world this would not be so. But we have not yet experienced a sane world. Crazy though our existing world may be, where there is time enough for thought and discussion, and the situation is not a major crisis, killing one another is not necessary. Left alone, even the most troublesome among us will eventually fall over dead.