When Harry cursed Malfoy with Sectumsempra, not many people were expecting him to bleed to (near) death in the middle of the flooded lavatory. It was the first time that Harry had used an evil spell and meant it, which is why when he tired to Crucio Bellatrix in TOOTP, it didn't work. But in Harry's defence, he didn't know what the spell would do, but although seeing as how it was recommended 'for enemies,' he must have known that it wouldn't have been good. Thank goodness for Snape coming to the rescue because as much as people tend to dislike Malfoy, you probably wouldn't wish that death on your own worst enemies.
