I Have seen interviews where both Troughton & Colin Baker had wanted their Doctor to be darker. McCoy and Ecclestopn seem to be heading that way but never got their. Tennant had a few dark stories but again just never seem to be an over all story line. I don't see Smith as being playing the Doctor dark. He is a mix of Tom Baker, Davison and Tennant (which is what a Time Lord is). With all the Doctor has been though I would think his darker side would show. I don't mean evil, just darker.