As we know, J.D. Lee was excommunicated and executed for his role in the Mountain Meadow Massacre. He was then posthumously reinstated in the 1960’s. Doesn’t that mean that he has been forgiven all his sins and eligible for the Celestial Kingdom?
The Church of Jesus Christ of Latter-day Saints does not have, nor has it ever claimed to have, the power to forgive sins. That belongs solely to the Lord Jesus Christ. The scriptures are very clear that the Lord will forgive whom He will forgive, but we are required to forgive all men. So forgiving sins belongs to the Lord, but forgiving all those who wrong us in some way belongs to us. The Church can forgive those who transgress against it, just like all people can and should forgive those who hurt them.
For those readers unfamiliar with the Mountain Meadows Massacre the Church has addressed it officially here.
The Mountain Meadows Massacre was wrong and should never have happened. But it did, and the Church has been dealing with the fallout ever since. All those responsible will have to face God and account for their actions.
The Church also has a commandment to provide the ordinance work for everyone who has ever lived. We can’t know if these people will accept it. We can’t know if they are worthy. We can’t know if the Lord has chosen to forgive them. We can only provide the ordinances.
We do not know how the Lord views J.D. Lee. We do know that his descendants, in doing their family history like they have been instructed to do, found that he needed to have work done and worked to make it happen. The effort of the Lee family is very much parallel with what each of us has been asked to do.