From Homestar Runner Wiki
Killingyouguy is Perducci's assistant and "secret weapon", and Dangeresque's "doom". However, he also seems to assist other villains as well, such as Uzi Bazooka. He is played by Strong Mad, but credited as "Himself". He has cut and bullet hole scars in the form of Sticky Notes. For the scenes where he is in shadow, The Cheat actually paints him black.
Killingyouguy is quickly knocked out of commission by Dangeresque Too in Dangeresque 1: Dangeresque, too?. However, in this scene, Killingyouguy is not played by Strong Mad, but by a stunt double, Strong Sad. Regardless, Killingyouguy returns as Perducci's assistant in Dangeresque 3: The Criminal Projective, and also seems to be working with Uzi Bazooka and Craig.
 Complete Filmography
Dangeresque 1: Dangeresque, too?, Dangeresque 2: This Time, It's Not Dangeresque 1,
Strong Bad: Dangeresque