With what I am about to say, I probably should start with an explicit statement that I accept Joseph Smith as a prophet and am amazed at what he was able to accomplish. I love him largely because he wasn’t the caricature everyone (his defenders AND his critics) tends to paint.
With that said, I find D&C 121 to be absolutely fascinating as it relates to "US", not just as it relates to "THEM". I believe we water down its message greatly when we read it in context of "others" and fail to read more closely and critically in order to see how it relates just as meaningfully (actually, more meaningfully) to ourselves and those with whom we worship.
D&C 121 starts, in my opinion, with Joseph finally losing it and demanding that God come down and wipe out the enemies of the Church. In a very real way, I think he “broke” finally - and fully gave in to his natural tendencies, if you will.
When you read the first part of D&C 121, in essence, Joseph begged God to be the Old Testament protector / destroyer God – not the long-suffering God of the New Testament that he largely had been up to that point. (That also fits with how I see Joseph’s prophetic role – MUCH more as a classic OT prophet than as a classic NT apostle – and I’m fine with that, since he obviously believed in a restoration of all things that was founded in OT theology in many ways.)
D&C 121 then follows his plea with a reassurance from God that basically tells Joseph to chill out and continue to endure – in a very real way, although couched in gentle, loving terms, telling him he had crossed the line and asked for that which couldn’t be granted. For what had he asked? A demonstration of divine power that would prove Joseph’s claims once and for all - which, in and of itself, might not have been such a bad thing, but, in context of killing numerous people, was a very radical demand. (I have used the word "demand" twice so far in this post, and I really do read it as a demand - not a simple, humble request.)
The middle part of the section then outlines the nature of the Priesthood and the responsibilities of those who hold it – and the section ends with what I see as a deeply reflective, humbling admission and/or divine admonition. It says (with the EMPHASIS mine):
v. 37 – “when WE undertake to cover OUR sins, or to gratify OUR pride, OUR vain ambition, or to exercise control or dominion or compulsion upon the souls of the children of men, in ANY degree of unrighteousness, behold, the heavens withdraw themselves; the Spirit of the Lord is grieved; and when it is withdrawn, Amen to the priesthood or the authority of that man.”
v. 39 – “WE have learned by sad experience that it is the nature and disposition of ALMOST ALL men, as soon as they get a little authority, as they suppose, they will immediately begin to exercise unrighteous dominion.”
Personally, I think one of the reasons Joseph received this revelation was to teach him (and we who read it) that even prophets (as part of almost all men) not only could cross but actually had crossed the line into unrighteous dominion – and, in doing so, could lose the protection of God they had enjoyed previously.
In a very real way, I see Joseph, not Brigham, as the modern Moses (and Brigham as the modern Joshua) – a very complex man who did great and marvelous things but who, in the end, was kept from leading his people into the promised land because his ego got the best of him and let his power go to his head. I absolutely love Joseph, from what I have read and felt about him – but he was a man, not a God, and I believe his death, while not justified in any way, absolutely was influenced greatly by his actions toward the end of his life.