I will add this:
I know a woman who is now a school counselor. Prior to being a school counselor she was an LAPD police officer. She had to fire her weapon 3 times in the line of duty, and each ended with fatal wounds to the suspect.
The first time, her and her partner were going around the rear of a house in preparation of a search warrant being conducted. The homeowner was actually hiding behind a shed and got the jump on her partner. After the suspect hit her partner with a shovel and knocked him to the ground, he was prepared to hit him again when she fired 4 times, each hitting the suspect in the torso. She didn't hesitate, she didn't think twice about what she was doing, she just acted. She said that she doesn't really know what she was thinking when it happened, she just knew that she was not going to let her partner get hurt.
The second time she was responding to a domestic abuse call. After she knocked on the door of the residence she heard the scream of a child and made entry into the home. When she reached a back room she saw a woman who appeared to be unconscious on a bed, a young girl whose face was bleeding, and a large man standing next to the girl. When she ordered the man onto the ground he grabbed the girl and was about to slam her into the bed frame and the officer fired twice, hitting the man in the upper torso; again, fatally wounding him. She recalled this situation and remembers most everything about it. She said that as soon as she saw the girl things seemed to change. When the girl's life was put in danger, she acted.
The final time that she fired her weapon was after a pursuit. She was patting down a woman when the suspect went "ballistic" as she called it. The woman elbowed the officer and pulled out a syringe and as she made an aggressive move towards my friend, both she and her partner fired, killing the woman. In this situation my friend said that she just reacted, there is really no other way to put it.
Obviously, my friend has since retired. After the last incident that I mentioned she decided that she wanted to go into a more safe occupation since she had a family and a young daughter. She still owns firearms, and I will bet anyone that if put in another situation like any of the above mentioned ones, she would not hesitate to pull the trigger.