States in the South were not known for being especially lenient to criminals, black or white. In many southern states, capital and corporal punishment remained on the statue books well into the nineteenth century for offenses which many of us would consider relatively minor. However, even for crimes which, to this day, are still considered serious enough to warrant capital punishment, like first degree murder, punishment in the nineteenth-century South went beyond what we might reasonably expect. Particularly if the offenders were black slaves, like Ben and Smart, who faced a truly heinous punishment for murdering the presumably white William Maxwell.
Because local authorities thought their crime deserved a particularly “severe” punishment, they ordered the two men burned alive. Like many slaves who were considered dangerous or rebellious at this time, Ben and Smart, unfortunately bore the full brunt of the law’s more vindictive side. Despite the contemporary perception that execution of slaves was primarily by hanging, the truth is that slaves sometimes endured the most medieval of punishments for their transgressions, and not just through the private punishment of their masters, but by the hands of the state. The historical record reveals that most slaves were in fact hanged, but the unluckier ones might suffer any number of painful executions, like being burned alive, broken on the wheel, hanged in chains, or gibbeted.

Maryland Gazette, Thursday, December 25, 1800
Further Reading:
Adalberto Aguirre Jr. and David V. Baker, “Slave executions in the United States: A descriptive analysis of social and historical factors,” The Social Science Journal 38:1 (1999), 1-31. doi:10.1016/S0362-3319(99)80001-9