I think James Swallow has nailed the whole falling to chaos thing in the short story The Returned. Well the doom eagle is tempted by visions of the four gods and when the vison of nurgle comes to him and delivers his fate causing him to doubt himself and saying in your darkest hour at the hands or bile you called out for salvation and I was waiting, the doom eagle hardly even rembers and can hardly believe nor dispute such claim as false memories come spilling in. A very easy way to fall to chaos as chaos needs just that one doubt that one chink in the armor of contempt, to take root. Aka mass chaos betrayals in the guard and populace. The sickness of the soul that travels faster than any plauge.