Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, 'Crucio'
but he couldn't perform it properly, bellatrix said
Never used an Unforgiveable Curse before have you boy? You need to mean them Potter! - righteous anger won't hurt me for long
i don't really know where i'm going with this, but i think it must be important enough for JK to have mentioned this in her book, perhaps his magic ability is rising within him or something and just reached boiling point at this point! i don't know, i'm just throwing ideas out here!! what does anyone else think??