Board Thread:Spoilers!/@comment-16953943-20151105020931/@comment-6175354-20151105021714

I think, there is a thread about this already.

Theory.... Hook was the one to reforge Excalibur, and is now literally sharing the burden of being the Dark One with Emma. In a final test, to determine if Light or Darkness will win, Emma pit Hook and Gold against each other. If "evil" Hook kills "hero" Gold, then the Light will be snuffed out. If "evil" Hook spares "hero" Gold, then the Darkness will be snuffed out.