They knew what price this night’s work would demand of them, but they had lived as wizards; as wizards they would die, spending themselves against the evil they’d allowed to live.
What could they have done that allowed evil to live?
My thoughts on the possibilities
1. Inattention/complacency
Falling afoul of the risk of every sentry.
2. Misplaced mercy
Instead of executing he who would become the Lord of Carnadosa when the first caught him practicing black magic, they allowed him to live perhaps because he was young or did an excellent job of faking repentance or was a close relative of a highly respected council member.
3. Excessive tolerance
I've seen this on the fringes of the we just need to understand them movement. I've also heard it referred to as "so open minded your brains fall out". In this case Carnadosa is just another religion. The dark Gods aren't really dark, they're just misunderstood victims of the light God's propaganda. All God's are the same: they all have good and bad aspects.
4. Some combination of the above. They're not necessarily mutually exclusive. For example excessive tolerance and inattention could combine. Say for example the Carnadosian's were careful to restrict known sacrifices to animals and the occasional serial killer. They could play on the excessive tolerance by saying that we're not as bad as our reputation see we only sacrifice animals / the worst of the worst criminals while playing on inattention by sacrificing innocent victims in secret.