Here's one more thing to consider -- you are legally responsible for the final resting place of every shot you ever fire. If you intervene in a third party situation and end up shooting at the perp, then you are responsible for every shot you fire.
In most gunfights, the majority of the shots fired miss the intended target. If you get into such a gunfight, the perp will be moving and so will you. Chances are it will be in low light. It is much harder to hit a moving target than a stationary one, so there really is a good chance that one or more of the shots that you fire will miss the perp. If one of your shots hits an innocent, you are legally responsible and might face criminal charges, up to and including manslaughter.
Here's a practical exercise for you. For the rest of today, whenever your go into a new room or building or are walking down the street, try to find a safe backstop that would stop a bullet. Remember that typical wood-frame construction walls (both interior and exterior) won't stop a bullet. Stucco won't stop a bullet either.
I think you'll find that in most places you go, you'll have a very hard time finding a safe backstop. And your perp won't likely stand still in front of that safe backstop. That alone should give you great pause about willingly wading into a third-party conflict.