The bottom line is that the tin letter - whether authentic or not - has no significance. Its contents merely reflect the thoroughly discredited public statements made at the time by John Lee, an admitted mass murderer. There is nothing new in the letter, so its authenticity has no bearing one way or the other on the content of his long-known assertions. They were thoroughly rejected and not substantiated at the time, and nothing since then has added any credibility to them.
This episode makes a convenient opportunity for enemies of the LDS church to attack its foundations in yet another venue, but they have always been without evidence to support the John Lee claims. To the contrary, the entire life of Brigham Young stands in stark contrast to these unsupported allegations made by a mass-murderer trying (unsuccessfully) to save his neck.
You got that right. I just felt the need to post a contrasting article to the one previously posted that started out with condemnation and only towards the end made a back-handed attempt at fairness. So, here it is for what it is worth.