He needs someone who won't stand against them and will adhere to their goals (aka the freedom of humans), the other option would essentially be mass genocide of the entire species, it's a sound reason not to kill those 2.Seems like page 18 is foreshadowing the two beastkins who repeatedly spoke up against this plan might avoid the worst of Light’s retribution.
Ah, the mangaka is a man of culture.I wonder what was the mangaka's line of thought on page 7