Snape. See the Lightning Struck Tower. Also for his pettiness and general bullying of Harry and Neville and no doubt countless other students. Yeah, James was pretty awful to him, but GET OVER IT! Picking on defenceless students is just cruel.
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"I thought she were a sandwich, you know, 'till she went spare on me hand" -- Ringo Starr (in a luvley Liverpool accent!!!) Help! (the film)