I you look at Delita's methods, sparing Olan's life was completely out of character. When considering someone like Delita the fact that Olan is alive doesn't mean that Delita didn't kill him, but that Delita actively chose to keep him alive.

The fact that Delita couldn't keep Olan safe from the church is no huge surprise.

He became what he fought against - a cycle of power where the most cunning lie, cheat, and backstab their way to the top.
Or he could have been fighting against the cycle where the peasants get swept up in sacrifice for the power games of the nobles. In that sense he succeeded completely.

Maybe Delita was a good king (as the prologue suggests) maybe he wasn't, history is full of lies!
Though we can never be sure given the lack of material we have to work with, but Alazlam Durai is certainly not a source that would be swayed by the popular interpretation of historical events. He probably has a very clear picture of the full implications of Delita's rule.