Summary:

Why this matters:

• The remaining Smith family accused Brigham Young of  having Samuel murdered, although his official cause of death is "bilious fever".
• William Smith, the brother of Joseph and Hyrum and a former apostle (before Brigham Young had him excommunicated), published two newspaper articles about this tragedy. The first stating that his brother Samuel was poisoned and the second pointing an accusing finger directly at Brigham Young:

"I have good reason for believing that my brother Samuel H. Smith, died of poison at Nauvoo, administered by order of Brigham Young and Willard Richards, only a few weeks subsequent to the unlawful murder of my other brothers, Joseph and Hiram Smith, while incarcerated in Carthage jail. 

"Several other persons who were presumed to stand between Brigham Young and the accomplishment of his ambitions and wicked designs, mysteriously disappeared from Nauvoo about the same time, and have never been heard from since." -William Smith, "Mormonism," letter to the “New York Tribune,” 28 May 1857

• Samuel's wife Levira also accused Brigham of having her husband killed. She also stated that Samuel  realized he had been poisoned. "[He ]spit it out and said he was poisoned. But it was too late--he died."
• Samuel's daughter Mary also accused Brigham of having her father killed. 
• Samuel was violently opposed to Polygamy and thus a threat to Brigham Young and his vision of the church, so Brigham had motive.
• Brigham denied from the pulpit any knowledge of Samuel's suspicious death, though he certainly profited from it.