Book SpoilerHarry is the final horcrux but they do not know that yet in the movie. Thus at the moment they only think they are searching for 6 horcruxes. When Voldemort killed Harry near the end he killed the horcrux part of Harry. In the end it was because Malfoy "owned" the elder wand by disarming it from dumbledore and malfoy's wand that caused the elder wand to backfire when it was used against it's owners wand finally killing him since all the Horcruxes were destroyed.
Or so is my understanding