That’s a good question. I don’t have an answer. The writers have purposefully kept Ben’s true nature ambiguous (they like to toy with the audience…like prey).
All I can say is that the monster is a bad judge of character. It killed my man Eko.
It seems to me that Ben was looking forward to being judged by the Island. Guilt can be a powerful force.
I actually think that leaving him live and continue for the rest of his life knowing that his decisions lead to Alex’s death was a harsher punishment than death. The fact that he also have to obey to Locke is a double punishment for him 
Could be that Ben, while not good, still has a purpose he needs to fulfill. Speaking of Eko, we did see Smokey spare him once.
But not sure why Smokey didn’t kill Keamy et al at the end of S4. Did they need to live to serve a purpose? Or did they have some anti-Smokinator that saved them?
Actually, in mythology a person’s sins are weighed on a balance at death. So maybe Ben is slightly in the positive right now and is allowed to live. And maybe Keamy and his crew were basically good people until the Sickness got to them and sent them on a killing spree?
I just watched the ep a couple of hours ago, and I got the feeling that “Alex”, who I think was just an avatar of the island/smoke monster still has use for him, but he’s no longer his mother’s favourite son. Locke’s taken his place.
This explains his actions in past episodes and was probably what happened between him and Charles Widmore.
I think Ben actually did regret the death of Alex and that regret was all that saved him.
Smokey killed Eko because he didn’t regret what he had done (he said that flat out) even though his actions indirectly caused his brother’s death. The Monster seems to approve of actual contrition.
Amazingly, I wept for Ben when he was shown those visions of Alex. Is this the first time we’ve ever seen him show true remorse for anything? I think it is. I think that’s why Smokie (Cerberus) spared him.
I have pretty much always maintained that Ben is the good guy for the show. He does some very bad things but i think he does them for the right reasons.
Similar to Tori, I think he is just very misunderstood…
I’m with Tom on this one. The writers and Michael Emerson have done an amazing job keeping Ben’s nature from us.
I agree that he truly did regret what happened to Alex, and that helped his case with Smokey. However, when I am watching the show, I never believe a word that comes out of his mouth; he blurs the lines between truth, half-truth, and lie so well, it’s a crap shoot. Again, brilliant writers/actor.
I also agree with Tom regarding Eko. Another great character. I understand Eko did not regret what he did, but he did take responsibility for the fact that it happened. Which is obviously not good enough for Smokey, and something Ben rarely admits to.
I think my feelings on Ben’s trustworthiness can be summed up in two lines of dialogue:
Jack: “Is [Ben] telling the truth?”
Eloise: “Probably not.”
I didn’t vote because I didn’t see a choice I could support. I don’t see that Ben’s redeemed himself, not yet at least. I think if he really gets Sun to Jin, I might feel differently. I’m dying to see the Miles episode next week so we can discover what the deal with dead people is finally.
I love Michael Emerson.
I love to hate Ben. (The Desmond beatdown was an oh-so-awesome treat, but not as good as the end of S3 Jack beatdown)
I actually got a little choked up during the Smokey/Alex home movies scene, it was the first time Ben has ever shown real emotion, and I wouldn’t have believed it except Emerson managed to portray it so well that we all got the message: Ben ACTUALLY REALLY REALLY DOES feel bad about Alex.
Bravo.
Interesting comment. I say to much bad. Smokey has just not finished with Ben yet.