Why is Harry Potter affected by the killing curse in the forest if he is the master of the Elder Wand and the wand refuses to kill its master?

Answer 1

The Elder Wand's will is absolute and will refuse to harm it's true master in anyway, but it will cast whatever spell it is commanded to.

After seeing Snape's memories, Harry left Dumbledore's office and went Into the Woods with the expressed intention of allowing Voldemort to kill him. When the wand had cast the killing curse, it did not affect Harry. It, however did affect the part of Voldemort's soul which was inside Harry at the time (which the wand would not have identified as its master). As their souls were connected though, the killing curse could have affected Harry in a way the Elder Wand didn't intend, but still it would be impossible for it kill him. Harry was either knocked out, or entered some sort of limbo with which he had an option whether to die or return to the living world.

This also explains why Voldermort was considerably weakened when casting the curse as he was in close proximity of his unintentionally created Horcrux enough to feel it when it was destroyed. This also explains how the Cruciatus Curse had no effect on Harry when Voldemort cast it to prove he was dead. It did however mimic the physical effects without Harry experiencing any pain.

Note: We know nothing for sure about the nature of the Elder Wand, its being or not being faithful to its master is only a theory.