Lawyers for an Oklahoma reserve sheriff's deputy who killed an unarmed suspect lying face-down on the ground and being restrained are expected to argue that the victim's drug use and health could have contributed to his death.

Robert Bates, a 74-year-old insurance executive who moonlighted as a reserve Tulsa County sheriff's deputy, is due to stand trial Monday on a second-degree manslaughter charge in the shooting death of Eric Harris last April.

Bates was playing a support role in an illegal gun sales sting in which Harris fled.

Body camera video shows that after deputies caught up to Harris and were subduing him, Bates yelled "Taser!" before firing a gunshot that struck Harris near his armpit, killing him. Bates later said he thought he was drawing his stun gun.