When taking my test to get my CCW permit, the issue of when lethal force is legal and when it is not was covered in some death. To legally use lethal force, the armed citizens needed to be reasonably sure (as determined afterwards by a jury), that either the shooter or an innocent bystander was in imminent risk of being killed. This reasonableness covered the facts as the shooter knew them at the time, even if subsequently they turned out to be in error. For example, if an armed perpetrator had fired his last bullet and his gun was therefore empty, but the armed citizen did not know that, then the armed citizen could legally use lethal force against the armed perpetrator. If an armed perpetrator has fired a shot, even if they missed, or threatened to shoot, then the armed citizen can legally believe the armed perpetrator is telling the truth, and may render the armed perpetrator incapable of taking offensive action. Dead armed perpetrators cannot take any offensive action.
If someone murders your child, but then drops their weapon and runs away, you cannot track them down to exact vengeance. But if someone has murdered your child and is still standing there with a weapon in hand, you may legally use deadly force to make sure that the armed perpetrator harms no one else.
Using lethal force to end someone's life, even if the person needed to be killed in order to prevent even more deaths, is a violation of God's Law, and is therefore an act of sin. But, permitting someone to murder even more people when one has the means to prevent them is aiding and abetting murder. I do not wish to speculate on what God's reaction would be to someone choosing to end one life, or to aid and abet someone who is taking many lives. But, I have no doubt that God's mercy would prevail.