Steve Harmon and James King are both charged with the felony murder of Alguinaldo Nesbitt. "Felony murder" is a murder committed during the course of another felony crime, in their case, the robbery of Nesbitt's convenience store.
It seems strange to many readers that Steve and King would be on trial for the same crime, when King was clearly the one in the store, with the gun. In fact, the prosecution is not even trying to prove that Steve shot Nesbitt, just that he was involved in the robbery that left the man dead.
Still, Steve certainly feels enough guilt throughout the novel to suggest that his choice to act as a lookout for King's robbery scheme directly lead to Nesbitt's death, at least in his own mind.