The reason Avada Kedavra and Expelliarmus met in the middle and backfired on Voldemort was because Harry was using Draco's wand, which he captured from him and Voldemort was using the Elder Wand, which rightfully belonged to Draco. Draco had disarmed Dumbledore at the end of book six, which made the Elder Wand rightfully his, even though he didn't take it. Two wands that once belonged to the same master won't act against each other, so the spells met in the middle and the stronger of the two (the killing curse) was the one to backfired, killing Voldemort.