Why did they decide to eliminate Mamoru?
Before trying to eliminate Mamoru, the committee had already gotten rid of a girl named Reiko, who belonged in the same group as Saki and the others.

The reason they killed her was due to her poor control of Cantus, out of fear of Fiends and Karma Demons. In this particular case, it's the fear of Karma Demons and their inability to control their Cantus, leading to unconsciously and involuntarily destroying their surroundings.

Mamoru had similarly weak Cantus. Though not quite as bad as Reiko's, the committee feared he might become a Karma Demon because of this.