Did he just? Yep, yep he did. Malfoy just tried to kill Harry in Hogwarts, outside of Dumbledore's office at the end of the Chamber of secrets. Harry is a twelve year old child who just outsmarted Lucius by making it look as though he just presented Dobby with a sock, resulting in his freedom from the Malfoys. But does that really warrant a death sentence? You can hear Malfoy mutter the words 'avada..' but is stopped when Dobby defends Harry and sends Malfoy flying on his arse. But what would have happened if Dobby wasn't there to protect Harry? Potentially, Lucius would have gotten away with murder, or he would have been spending the rest of his days behind bars in Azkaban. Either way, using the Killing Curse is a bit much for losing the House Elf who you hated in the first place.
