When I come out of the barn at the very end, for my very last stand, can I kill any of the main culprits who I'd like to exact revenge on?
To disagree with some of the other posters, The Last Enemy That Shall Be Destroyed mission on the Red Dead Redemption Wiki offers the following as a possibility.
Even though the player can shoot Edgar Ross and Archer Fordham dead in the final Dead Eye moment, Ross will still be alive in the following cutscene.
No. you cannot. Unfortunatly, there are to many soldiers around, and they are hiding behind them. Even with the Evans repeater and moonshine along with a maxed dead eye means 22 dead bodies, all soldiers. The Federal Agents do not poke their heads up until after John gets gunned down. I have done the mission twice, and neither time was able to kill anyone other than the soldiers. It is fun taking moonshine before walking out and using the Evans, and just blast as many as you can. A bit gratifying, but the mission always ends the same for me.