[woodguy]

Lots of spoilers



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Why was Draco Malfoy the only one who could control the wand?

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Its not that he was the only one who could control it, it was that he was the actually owner of the wand since it was he who blasted it from Dumbledore's hand, and not Snape.

Voldemort assumed Snape was the owner of the wand since he killed Dumbledore, but since Dumbledore died voluntarily and didn't have his wand in his possesion at the time, the ownership of the wand never transferred to Snape.

Since Voldemort was never the "true" owner of the wand, as he assumed he would be after killing Snape he could not maximize its potential.

Harry was the true owner, since he had taken Malfoy's other wand. </font>