A couple of things that apply to Florida law....not sure about other states:
- use of lethal force in self-defense would include the life of another person. In the end you will have to prove that you or the other person were in fear for your/their life. That's a harder one to prove if it's someone else, since you don't know what they will say. There have been instances where someone has defended the would be killer after they were shot by a bystander.
- you cannot hold someone until the police arrive. A civilian does not have the power to detain someone for a crime. You can only defend your life, or another's life.....