This is a tricky one. There are a couple side quests that you need to complete in a greedy fashion to unlock this achievement in Chapter 3 the protector as well as turn down the offer to form a truce. For the side quests, you'll need to sell the sheriff's badge and the peoples land titles. You can sell the star at a general store and the land titles at a bank.
Also, since unlocking this achievement will
making it impossible to get the Getting the Gang Back Together achievement/trophy I put together a video of how you can easily complete this achievement and then go back to a previous save and not have to redo a huge part of the story.
Hope this helps!