Robert Stewart, 47, has admitted he was the shooter who went on a killing spree March 29, 2009 at the Pinelake Health and Rehabilitation Center in Carthage.
He faces eight counts of first degree murder - one for each of the seven elderly patients and the nurse he gunned down at the home.
Prosecutors have focused on the victims, residents Tessie Garner, 75; Lillian Dunn, 89; Jesse Musser, 88; Bessie Hedrick, 78; John Goldston, 78; Margaret Johnson, 89; Louise DeKler, 98; and nurse Jerry Avant, 39. All were shot in close range - most sitting in their wheelchairs.
Nursing home staffer Wilean Fletcher told the jury Tuesday that she and nurse Avant were side-by-side checking on residents when Stewart confronted them both with a shotgun.
She said she took off to the laundry room and Avant ran for an exit when he was shot and later died.
Fletcher said when things settled, she entered the secured Alzheimer's unit and ran into Stewart's wife - who had left him weeks earlier.
She said Wanda Luck told her, "I believe that's my husband, I want to see if that's him."
Stewart was on the ground from a gunshot wound to the chest - taken down by Carthage police officer Justin Garner.
Fletcher said she and Luck walked by Stewart and Luck said, "Oh my God, it's him, that's him."
Prosecutors say Stewart was trying to get back at his wife and knew what he was doing.
However, the defense told the jury in opening statements that the evidence will show Stewart had been taking Ambien for two years and 48 hours prior to the shootings, a nurse practitioner had prescribed Stewart an anti-depressant and Xanax - a dangerous combination of drugs that the defense says caused what happened that day.
Prosecutors plan to seek the death penalty if Stewart is convicted.