Kylo Ren is a title, The Adam Driver character is the first Kylo Ren, heading up the First Order, Luke comes out of hiding to destroy the First Order, but realizes he can't destroy it, it will just come back, instead he has to rule it, and so he takes over, putting on the mask.

JJ Abrams has stated that both sides of the force think they are the good guy, so this sets up an interesting contrast, and allows Luke to be converted to the darkside with a well intentioned, but anger-fed motivation.

Alternative theories must account for why Kylo Ren is seen with and without a mask, my theory explains that the masked Kylo Ren is Luke. Further, my theory explains the apparent lack of Luke on the poster. There are two Kylo Ren's, or perhaps the original advertising and merchandising has been misnaming the character to keep his identity secret, and we'll get a Luke Ren, or maybe Kylo is an anagram of Luke.

Also, Star Wars likes to have big reveals that instantly change the way the character is viewed, Luke being a bad guy would accomplish this, and set up the villain as a familiar follow through from ep 4-6. If we are to believe that Disney has produced a quality story, than we can assume we will get a good villain, not some no one out of left field like General Grevious or Count Dooku.

George lucas has stated that the next 3 would be 'what happens to luke skywalker' and frankly, if he becomes a benevolent old professor/ mentor type, there would be no tension in the plot, luke's character must develop and as ep 6 ended with him being the all around great guy, he's going to have to become flawed, which again sets up great drama