I can't find any reference where Bigler implicates Young directly in the massacre.
Eliason:
Nevertheless, Forgotten Kingdom strongly hints, without providing any new evidence, that Brigham Young was not only involved but was a direct instigator. Bigler points to a meeting of Piede Indian chiefs with Brigham Young a week before the attack; Piedes were later known to be among those involved in the massacre. Bigler also refers to Brigham Young's instruction not to harm the Fancher party as an "alleged" order. This loaded term "alleged" is not applied by Bigler in any discernibly evenhanded way and appears rarely throughout the book and never in conjunction with any questionable action of any gentile. The word's use here seems designed to prejudice the reader against Brigham Young and to suggest that the memory of this instruction was fabricated after the fact to protect the church president (p. 170).(My emphasis)
In this meeting with the Piede chiefs (actually it was Hamblin, not Young, who met with them) BY asked for their help in the "war" against the United States government, or they "will kill us both". Eliason uses the words "strongly hints" and "seems", but I see no direct inference that Young was directly involved.
Even Jerald Tanner observed, quoting Bigler in an article:
Whether or not Brigham Young directly ordered the massacre may never be known. However, he seemed to have no problem with the bloody deed after the fact.
Quoting Brooks:
Juanita Brooks observed:
While Brigham Young and George A. Smith, the church authorities chiefly responsible, did not specifically order the massacre, they did preach sermons and set up social conditions which made it possible.... Brigham Young was accessory after the fact, in that he knew what had happened, and how and why it happened. Evidence of this is abundant and unmistakable, and from the most impeccable Mormon sources.
Knowing then, why did not President Young take action against these men?... He did have the men chiefly responsible released from their offices in the church following a private church investigation, but since he understood well that their acts had grown out of loyalty to him and his cause, he would not betray them into the hands of their common "enemy."...Someone assuredly warned all the participants, so that for many years they were all able to evade arrest.
The church leaders decided to sacrifice Lee only when they could see that it would be impossible to acquit him without assuming a part of the responsibility themselves.... this token sacrifice had to be made. Hence the farce which was the second trial of [John D.] Lee. The leaders evidently felt that by placing all the responsibility squarely upon him, already doomed, they could lift the stigma from the church as a whole. (The Mountain Meadows Massacre, p. 219-220)
Eliason's conclusion:
Despite these problems, Forgotten Kingdom does make some important contributions. As Bigler rightly suggests, a chronicle of the establishment and dismantling of Latter-day Saint theocracy in the American West is long overdue. Many Mormons' historical consciousness stops in 1847 as if the arrival of the pioneers in Utah were the end of history. Bigler invites us not to ignore the fascinating 1847—96 era. For this we should thank him. However, there are some signs of this era's reemergence as an important time period in LDS historical consciousness. At the September 1999 fundraiser for the Association for Mormon Letters, keynote speaker Richard Bushman suggested that because of our experience with federal intervention and domination, Mormons now exist in a state of mind that shows many features of a postcolonial condition.31 Drawing on the work of Palestinian scholar Edward Said, Bushman described ways in which colonized peoples begin to accept the image of themselves constructed by their colonizers.32 Said and Bushman invite us to be cognizant of this colonization of our minds.
The fact that many Mormons today fail to celebrate our ancestors' courageous, principled, and amazingly well-disciplined nonlethal defense of local autonomy, noncontentious governmental operation, communitarian living, cooperative economics, personal religious freedom, and family privacy—and instead shamefacedly avoid engaging with our theocratic past—may indicate that we have internalized the ideology of our colonizers. David Bigler's stirring the coals of this secret-shame-that shouldn't-be is a wake-up call to those who engage in Mormon studies to rise to the challenge of appreciating the historical meaning and current implications of our theocratic past.