Malfoy asks Fleur to the Yule Ball, and once she declines, he starts insulting her. Harry gets involved and the argument escalates, ending with Malfoy dead. In what can be described as a bonding experience, Harry and Fleur hide the body together.
All seems well for a week, but then Crabbe stumbles across the body. Having witnessed this from some Tower where they were having a moment, Harry and Fleur know what they need to do. However, a few days later, Goyle checks out the same patch of Forbidden Forest, and therefore he also needs to disappear...
Or alternatively, how Harry and Fleur won the war with a series of preemptive murders because they wanted some time alone, but people kept finding the ever increasing number of buried bodies nearby.