In this world of daily bloodshed and bodies piling up around you, it was still a bit much for Heath to take in when confronted with the need to kill one of Negans Saviors whilst they slept. Everybody has seen horrible things, everybody has done horrible things, but killing a man in his sleep is vastly different to killing an undead sort whos out to chew on your flesh. As such, Glenn realises the conflict going through Heaths mind and steps in to help out his pal, meaning that any blood is on Glenns hands rather than Heaths.