That's right. It's not just candidates that he can't kill. He can't kill ANYONE. Not a single living person.
But but but, we've seen him kill! Have we? Or have we been lead, by the smoke monster, to believe we have? The SM can manifest itself as the dead. We've watched it act out new death scenes (like Richard's wife) for the benefit of manipulation. Whose to say that every "murder" committed by the monster wasn't just part of the grand scheme to convince these living people to off themselves.
Until a very short time ago, the smoke monster didn't even have the highest body count on the show. I'm still not convinced he does. It seems rather a simple trick to him to get others to kill each other.
First up, Seth Norris. The pilot was dead in the pilot. Who "woke up" in the cockpit was actually smokey, pretending to be Norris. All the SM wanted to do (or could do) at this point was give them some false information regarding their location. We've since been shown that they weren't 1,000 miles off course. The monster "killing" seth was just for show, just like he "killed" richard's wife. (for show) Even all the temple deaths were redshirts, and quite possibly all already dead. Eko is another good example of someone who should have been dead, but smokey tricked us into thinking he lived a couple more weeks than he did. (Did the swan event or the polar bear kill him? I think so)
Every "confirmed kill" by smokey could very well have been a long planned illusion. He might be the biggest scariest bad in the history of television, but he might also be harmless apart from his considerable power of persuasion and illusion.