Why did Lord Voldemort's curse backfire on Harry Potter?

Answer 1

He was reduced to a powerless thing. The spell rebounded because Lily sacrificed her life, died for Harry, although she had the choice to live. Love is the strongest magic there is, and Lord Voldemort knew nothing of it because he had never been loved.

Answer 2

Lord Voldemort offered Lily Potter a choice, if she stepped aside he would spare her life and then kill her son. Lily refused to do so and died to protect her son. This evoked powerful magic that Voldemort never understood.

The protection meant that Voldemort could not harm Harry, and so when he tried to kill him the curse backfired. It would have killed him had Voldemort not already created Horcruxes. Instead a piece of Voldemort's soul attached itself to the only living thing, Harry, and the rest of him was left without a body and extremely weak.

Answer 3

When Harry was a baby the killing curse rebounded because Lily Potter sacrificed herself.

In the Deathly Hallows the curse rebounded because the Elder Wand's allegiance was to Harry, so it a) didn't work for Voldemort, and b) couldn't kill Harry.

Answer 4

because harry was actually the owner of the elder wand and the wand can't kill its owner. so the curse backfired and killed voldemort.

Answer 5

the wand doesn't want to attack its master, so it rebounded.

Well, the Elder Wand had already betrayed its previous masters many times, so it cannot be that simple.

Answer 6

at harrys house, godrics hollow