One of two voices that Chara and Frisk occassionally hear when they die. It's implied the Anomaly is overseeing Frisk and Chara's journey. The Genocide voice urges them to try and kill the other monsters, adopting Flowey's philosophy, and butts heads with Pacifist. Would be represented as a red spotlight in a musical stage version. 

Any range or voice tone is good... after all, it's you. (Just don't try to sound too evil—remember, the best way for a villain, even a minor one, to get someone on their side is for their side to seem like a good option)