A little bit of canonical evidence to discount this theory. First, Luke appears in Shattered Empire #4 acting very un-Sithlike. This is post-ROTJ, so we now have to discount official story. And if he is Sith and is still “lying through his teeth”…to what end? If he’s now the most powerful Force sensitive being in the Universe, it’s his for the taking, right?
Second, Force ghosts. Anakin, Ben, and Yoda are all seemingly quite proud of Luke at the end of ROTJ. It’s already been established in Attack of the Clones that Force ghosts would have been able to sense Luke’s descent into darkness (Qui-Gon’s “Anakin…no!” voice over while Yoda is meditating). We would have to assume that Luke is hiding his darkness from beings that are now “one with the Force.”
And now some more speculative evidence and rebuttals. We can’t use “Yoda was never wrong” as supporting evidence here, because he admits his own blindness regarding creation of the clone army and his failure after Palpatine’s exposure, Anakin’s turn, and the fall of the Jedi. Making him the total authority on all things light and dark can’t be done.
To say that Luke turned to the Dark side solely to defeat Vader and call that a plot hole is misreading the entire scene. Vader’s threat against Leia is enough to provoke enough fear and anger that causes Luke to lose control, which he regains after he sees how close he is to following the same path as his father. I don’t read this as a plot hole at all.
Mark Hamill’s ideas about what would be “fun” to play as an actor can’t seriously be taken into account in this sense. He can project his own sense of darkness into his acting, but that doesn’t alter the story.
These final scenes may be underwhelming for some, but I think we’ll have to chalk that up to dissatisfaction with George Lucas (and is that really so un-common a thing?) rather than some hidden plot line that will suddenly resurface after 30 years.
I will not discount the possibility that Luke has since turned to the Dark Side in the span of the 30 years that pass between Endor and “The Force Awakens.” But I firmly believe it didn’t happen in this film, and hasn’t happened in current canonical stories. If it has happened, it’s happened afterwards in something that has yet to be released or even written.