Near to the end of Harry Potter and the Chamber of Secrets, when Dobby is set free through Harry's trick of hiding one of his socks in Tom Riddle's diary that Lucius Malfoy then gave to Dobby, Lucius gets quite angry at Harry for tricking him in setting Dobby free and pulls out his wand to attack.
Just before Dobby sends him flying, Lucius is about to cast a spell and it sorta sounds like he says "ava" before he is interrupted. Given that he is a Death Eater and a follower of Voldemort, was he about to use the killing curse Avada Kedavra on Harry here?