Ceolmund is of the firm conviction that if you have to use underhanded methods to get people onboard with your plan, your plan is bad and should be discarded. This one of many reasons he wants to screw over the Sentinels.

His primary reason is that the Sentinels screwed him over in the name of ensuring the Prophecy when he was in a vulnerable position. (Ceolmund means protector, and it is a very meaningful name.)