That's not actually true. The colonial centurions wanted to become more human, but they were failing. The Five gave the centurions the power to create skinjobs, which ended the war. Under the guidance of the Five, the centurions and the skinjobs might never have returned to destroy the colonies, they may well have advanced to the level of the Earth cylons with the ability to give birth, hell they might have one day integrated with the humans. Becoming more human is clearly something cylons want - humanoid bodies, the ability to having baby. The Earth cylons succeeded and the colonial cylons probably would have too with the Five, but there was a fly in the ointment.
Cavil wasn't like the other children, he didn't want to be human, he hated being human, stuck in that miserable old body when he should be a perfect awesome machine. So he got rid of his masters, and murdered the beloved brother that was most like what he hated. After the destruction of the colonies, the other models began thinking they made a mistake, which lead to the New Caprica experiment, but Cavil has never wavered from his desire to wipe out all that is human. He calls it "justice" to convince the others, and perhaps himself, but that simply isn't true.
Notice how when Ellen asks the centurion for help out of the tub, and it extends a claw, before turning it into a hand? The centurions aren't used to being treated nicely. Cavil doesn't see them as "his people", they're "his tools". He was against removing the limitations on the non humanoid Cylons and those that were for it became his enemies.