Chaosium's BRP is, by far, the best horror game I ever played. The fact it is also my favorite roleplaying game is also an indicator of how much I like it. Still, Supernatural is less about horror and more about dark fantasy as fear is never an issue to the Winchester brothers. You may want to give a look at Witchcraft by Eden Studios. The core book is available freely as a pdf file and it will be easy to stat Sam (minor gifted) and Dean (mundane), as well as most of the monsters. Witchcraft uses the Unisystem engine, which also power several horror games, such as All Flesh Must Be Eaten, Armies of Darkness, Buffy, and Angel, although some using the cynematic version of the rules.