Watch Little Bill's conversation in the jail to the writer. It shows that Bill ran in the same social circles as Bob when he was younger and that there is little real difference between them. He also knows about Eastwood and his history.
The old gun fighters have grown up and are trying to find ways in the world but the world has no real use for them anymore.
Bod tries to become a posh English assassin (notice his real accent comes out when he is being driven away). At heart he is, basically, an East End thug but is trying to become something else and it falls apart, he is beaten, humiliated and driven out. The only part of him that still works is his gun skill (notice the competition on the train).
Little Bill was a gun slinger and is trying to become an honest man but simply does not have the skills. Notice he is trying to build a house (very common skill for a man at that point) and he is terrible at it, he is also not a very good Sherrif. His spineless way of handling the girl having her face cut up just causes trouble for the town. His bullish way of trying to stop assassins from arriving does not work. He beats Ned to death even after getting the information about who was with him, which directly causes Eastwood to come back and kill several people.
Eastwood tried becoming a farmer but his farm was failing, to make enough money to set himself up he has to resort to his gun skills and to handle that stress he resorts to drinking (notice how the first time he drinks is just before he rides back to town to kill Little Bill).
In a deeper analysis you can say that these men know each other as well as they know themselves as once you cut beneath the surface they are the same.