Sin-Eater

Sin-Eater was a name given to artificial beings created by the Hesguard Institute in the 53rd Century as part of an experimental process intended to rehabilitate criminals. Director Highsmith referred to the procedure as the Bad Wolf Process.

Negative emotions such as fear, anger, and hatred would be artificially transferred from the subject into the Sin-Eater, under the theory that removing such emotions and impulses would render the patient incapable of committing offences. Thus they could "move on" and subsequently be released into society. Unfortunately, according to the Ninth Doctor, the process didn't work- the patients would eventually experience a psychotic break and go on to commit even worse atrocities.

In appearance Sin-Eaters appeared as featureless humanoids, until the process was completed, then they took on the featured of the patient whose emotions they absorbed. Though originally designed as non-sentient receptacles for emotions, the introduction of the Doctor's unique mind gave them all a form of sentience, turning them into monstrous being driven purely by the dark emotions that had formed them. (COMIC: Sin-Eaters)

Behind the Scenes
Historically, sin-eaters were individuals who would eat a ritual meal near the body of a recently deceased person, symbolically taking the sins of that person into themselves to absolve the deceased of those same sins.