I believe good characters might follow Izrador is to avoid being killed, to defend their families....or perhaps to try to work with him in an attempt to try and subvert his conquering efforts.
Imagine an invasion by orcs that won....your army is destroyed, so all that's left are roving bands of mercenaries or defeated soldiers with little organization. They might turn to looting or banditry in order to feed themselves.....given they are now actively hunted by the invaders.
In turn the villagers/civilians who remain are now stuck. They find early on that when a few local toughs killed an orc or two, the orcs responded by slaughtering 10 villagers for every orc who was killed. And when more were killed in punishment for delivering food to the remains of their city's army that support dries up.
Soon, simply to avoid suffering more punishment, the civilians might actively stand against their own soldiers, hoping that the orcs will let them live in peace.
It's a pretty scary progression, and I think an excellent explanation of why good would assist evil.
You have to remember....in this campaign, the Shadow *won*. It's quite different from anything else out there.
Banshee
