Xunzi is the great dissenting voice inside Confucianism, and the one whose students carried it somewhere he did not intend.
Against Mencius, he argued that human nature is bad — or more precisely, that what is inborn is appetite and self-interest, and that everything admirable in a person is acquired. His image is the warped board straightened by the press, or the blunt metal made sharp on the whetstone: the result is real, but it is the product of work, not the release of something latent. Goodness is deliberate artifice, and the moral achievement is greater for it, not smaller.
From this follows an emphasis on ritual and institutions that is closer to constitutional design than to character formation. If desires are unlimited and goods are not, conflict is the default state of humanity, and ritual exists to apportion — to assign roles, ranks and shares so that competition is channeled rather than fought out. Ritual on this account is not decorative and not merely expressive; it is the technology that makes society possible at all, and the state is its guarantor.
Xunzi was also the tradition's hardest naturalist. Heaven, for him, is a regular natural order with no interest in human affairs: it does not send omens, drought is not a rebuke, and to pray for what one could get by working is superstition. This detachment of the natural from the moral is unusual in the ancient world and made him readable, much later, as a proto-scientific figure.
His two most famous students, Han Fei and Li Si, kept his account of human nature and discarded the ritual: if people respond to incentives and not to virtue, then law, reward and punishment are sufficient, and moral cultivation is a waste. That is Legalism, and Li Si became chancellor of the Qin state that unified China and suppressed the Confucians. Xunzi's reputation never recovered from what his students did with him, and orthodox Confucianism followed Mencius instead.