I don't think this song is directly talking about Jack and John Marston. I don't think it should be taken literally as in a man holding another man's gun. The two men are within yourself. It's the past you and the you now. "your hand's upon a deadman's gun, and you're lookin' down the sights" is when you are looking through the eyes of your past self.
"your heart is worn and the seams are torn and they've given you a reason to fight" is saying that you are finally tired of your old ways (whether it be that you are moving on in life, leaving a relationship, stopping drugs" and you're tired of the troubles your problem gave you. Your heart and soul are tired of being worn down by the issues you had.
"and you're not gonna take what they've got to give
and you're not gonna let 'em take your will to live
because they've taken enough and you've given them all you can give
and luck won't save them tonight
they've given you a reason to fight"
You are giving up the addiction or the loveless relationship. You're not going to give in to whatever promises or feelings are made because you are just gonna fade away if you stay. You've given your best efforts and all your efforts, but the problem or relationship isn't getting better.
I don't think this song is directly talking about Jack and John Marston. I don't think it should be taken literally as in a man holding another man's gun. The two men are within yourself. It's the past you and the you now. "your hand's upon a deadman's gun, and you're lookin' down the sights" is when you are looking through the eyes of your past self. "your heart is worn and the seams are torn and they've given you a reason to fight" is saying that you are finally tired of your old ways (whether it be that you are moving on in life, leaving a relationship, stopping drugs" and you're tired of the troubles your problem gave you. Your heart and soul are tired of being worn down by the issues you had.
"and you're not gonna take what they've got to give and you're not gonna let 'em take your will to live because they've taken enough and you've given them all you can give and luck won't save them tonight they've given you a reason to fight" You are giving up the addiction or the loveless relationship. You're not going to give in to whatever promises or feelings are made because you are just gonna fade away if you stay. You've given your best efforts and all your efforts, but the problem or relationship isn't getting better.
This is my interpretation.