Besides the obvious alternative that Bungie simply picked the song to give off a western vibe, I like to think that it's a way of darkening the scales. Guardians are nothing but killing machines, especially our own. This song seems to be an introspective look at how this second life has affected us, and how there is a bit of gray area between the Light and Darkness. Personally, I find it interesting that this song was picked for this trailer. Then again, maybe things are a lot simpler than we are thinking, but this is fun nonetheless.