When a deadly weapon is produced, during the commission of a crime, it is a threat to the life and limb of all present. The individual making the threat has already demonstrated a willingness to commit violence, and it's impossible to predict how far he'll go. Elimination of the threat is the only way to insure your immediate safety, to say nothing of future danger from a demonstrably violent individual.
As has been quoted, over and over, on this forum:
All that is necessary for the triumph of evil
is that good men do nothing. -- Edmund Burke