"Suddenly pull a 180" is the fallacy. If you don't count the clues that created the theory in the first place, then this story was set in motion fifteen years ago, when Zoro inherited the will of Ryuma on panel.
As for Oda's intentions, he's made it clear that Zoro is willing to kill and that he would opt to kill Kaido. Is more "brutal" than Luffy punching his female crew member in the face, or the face melting at Marineford that the anime wouldn't even show? We already know that they're willing to show a beheaded dragon, which is frankly a pretty humane way to kill someone. If you really want to believe that it goes against something Oda has installed, hey maybe Kaido will get better like the mountain god or Orochi. You know, the two recent beheadings.