But if one does use lethal force in what he believes to be defense of another, he better be sure he knows what's going on and that his use of force is legally justified. A mistake can easily send him to jail.
And conditions the person being defended may have created can than attach to you in determining if the force is justified.
Be VERY careful.
If you neighbor runs into your house bloody from a brawl his use of lethal force may not be justified, and then your use of lethal force in his defense might not then be justified.
It really pays to understand both the statute law and the case (common) law for the jurisdiction you are in, and have at least a passing familiarity with local politics (it often drives prosecutors).
The Bloomfield Press guides to state gun laws are a decent start.
Anyone considering using a firearm for self defense should have at least looked at them for their state.
They are not painfully complicated, and have decent citations to applicable case law.