Will I be prosecuted if I shot a potential risk guy against me?

There is no way to correctly answer your question. There is not enough info. Never think that any blanket statement is true when concerning the law (ie. you can only use deadly force when you are in fear of your life).

If you look at your states laws it will not say you can kill someone shooting at another. What it will more likely say is deadly force is justified when the actor reasonable believes it is immediately necessary to stop anothers use or attempted use of unlawful deadly force.

The wording is very important in every law.

If someone says that you can shoot someone in your scenario, I can think of an example of when it is not justifiable. So the blanket statement is not true. The person with the rifle may be using lawful deadly force on another to stop a crime.

So their use of deadly force must be unlawful.

Also your belief that you must use deadly force must be reasonable. Just being in fear of your life is NOT justification. What if you having a mental disease, actually believes your neighbor was sent from Mars to kill you. Even though you are in actual fear of your life, you are not reasonable in this belief. Thus your use of deadly force is not justifiable.

Depending on your states laws, you may be able to use deadly force when you are not in fear of your life. Like in your scenario, if he is pointing a firearm at someone else, you are not in immediate danger. But you can use deadly force to protect someone else.

This answer is based on Texas law (the state I know). You must look up your states laws to know for sure.

Not if there was a legitimate fear for your personal safety, that makes the act one of self-defence and hard to prosecute.

If someone is shooting at people in public then you would be legally justified to shoot them. If it is a fake or legally owned gun that is not aimed at anyone and you decide to shoot them then you will probably go to prison