Ok, i kinda think that Ben isn;t really back to his evil ways. here are some reasons for that
1.) Ben killed Widmore, or so he thought, before he was able to reveal very important information to the smoke monster. (he was able to cover it up by acting out "revenge" by why go through all that before killing him if that was his only goal?)
2.) He gave Miles the walkie talkie in case he "needed him later". If this was his plan (to sign up with smokey) why would he ever need miles? but if he was working against smokey, miles would prove a valuable asset.
3.) this was the 2nd time he was offered the island, he refused it before and nothing has really changed since then
4.) this kind of ruins his whole redemption segment, i mean why go through all the that just to have him "turn back"
I feel like this is yet another con from ben maybe he's still selfish and nosy, but this time he's doing it for the good of jack and co. I have a feeling he'll be back to good at some point in the finally. (although i do love good ol manipulating ben)