Ultimately its very likely that although Delita let Olan live, nothing says that he wasn't responsible for the Durai Papers being sealed away for such a long time before his descendent, Alazlam, recovered them and showed them to the world. It would have been in Delitas best interest for his immediate future to make sure that Olans story did not make it to the public, less his followers desert him over how he achieved his power.

Either way I believe the ends justifies the means, Delita may have been underhanded to reach his goals, but it's not like he went around killing innocents. The leaders and various people he assassinated were far from good people, with the exception of ovelia, which he may have done simply for his own survival (if she tried to kill him once, who says she wouldnt try again).

bottom line- Hero