Add the beans, bacon, onion, jalapeño, garlic, cumin, cayenne, oregano, salt, and pepper to the slow cooker. Pour in enough water to cover the beans, and then add another cup or two. Err on the side of having a extra since it's easier to take out water than to revive burnt beans.
Cover and cook on high for 8 to 10 hours, until beans are very tender. If water evaporates, add more. If it seems like there's still a lot of water once the beans are done, remove to a bowl.
Mash the beans with a potato masher, or blend with an immersion blender. If needed, add some extra liquid to help with the creaminess. If desired, add the butter too.
Taste and add salt and pepper if needed. Serve, topped with fresh chopped peppers, melty cheese, or cilantro.