Koster is an advocate of the death penalty, and as of July 2013, there were 21 inmates on death row in Missouri whose executions he was pressing the Supreme Court of Missouri to expedite. After the 21 inmates filed suit before the state Supreme Court against the Missouri Department of Corrections over the use of the drug propofol used in lethal injections, concerning cruel and unusual punishment, the state Supreme Court temporarily halted the further use of the death penalty until the case had been ultimately decided. With Missouri law allowing for the use of other forms of the death penalty in cases where lethal injection is not available, Koster has advocated the use of gas chambers to execute Missouri prisoners.