I would imagine Gunpowder would be an Anime, but if I had to pick somone... It'd be Andrew Jordt Robinson, for his voice and... personality. I like what he did for Garak (DS9) I know he's not the best but I don't think looking up "bored psychotic killer zealot actor " would lead to the search I'm hoping for.
Ooohohoho. I found someone better. Andy Morris, the one who played a psychotic jester who was in the Dark Brotherhood (Assassins) in The Elder Scroll 5: Skyrim.
His voice is absolutely perfect for when he is excited.
SPOILER ALERT
Also, Gimmepie, are you making this thread because you are rethinking making Gunpowder in to an Anime?