Well currently DC seems to be intent on aligning every property they have to fit their New 52 comic book universe. Recently they had a comic book event called Forever Evil where the [Justice League](movie:401267) was replaced by the Crime Syndicate who tried to take over the world. Lex Luthor actually went and got a team together to ultimately defeat the Crime Syndicate and gain the trust of the entire world as well as being exonerated of all crimes. Now Lex is running around with his new found hero status, trying to join the Justice League to stop a bigger threat on the horizon.
At the end of the day, Lex Luther more or less has humanities best interest at heart... somewhere, he believes humanity should be self sufficient and that relying on super heroes only makes mankind weaker. Lex is more or less, a man of the people and in this world, the people have built up a distrust of the Justice League for not being around when they were needed most, this is Lex's reasoning for joining the League, to restore the public trust in the team being as how he is Earth's Champion at the moment.
Now after the damage Superman left in his wake in [Man of Steel](movie:15593), is it possible for Lex Luther to join the League as opposed to being a main villain. I only say this because Man of Steel 2 is so far away that to keep Lex around, he would have to be more than just a 1 and done villain.