Why did the Dark Side Make Sith Ugly?

As force users immersed themselves in the dark side, they would begin to notice various ailments, ranging from minor ones like yellow eyes, all the way to major ones like the melting and cracking of skin.

Creator of the Rule of two, Darth Bane believed that the flesh and bone of physical beings did not have the strength to channel the sheer power of the dark side without causing damage as it passed through. While the Sith served as vessels for the dark side, the mental and physical toll required to command it to their will heavily corrupted them, and The deeper a force user delved into the darkness, the greater the effect.

Some force users like Dooku never allowed themselves to embrace the dark side tightly enough to become corrupted, and some extremely proficient dark side users could even ward off this corruption through the force concealment ability.