You're a student dating Hermione, and you're in the Battle of the Astronomy Tower. You've made it up the tower, where you're unnoticed. Bellatrix, Snape, Greyback, Dumbledore, and Draco are in the tower, and don't notice you. Due to that, you have a chance to assassinate one of the Death Eaters with no trouble by use of a hidden blade you put on beforehand, at the cost of the other three fighting you in retaliation