Candidates can be killed by most anyone or anything. It's just not something they can do to themselves (which is really confusing since Sawyer's actions shouldn't have had done a thing to hurt them; and who knows, maybe it wouldn't have if Sayid didn't run off with it), nor by Smokey. Ben wouldn't have any trouble killing them, considering he had no trouble doing it to Jacob.
Why Jack thought Sawyer's toying with it would change anything is beyond me. By his (and the established show's) logic, neither Smokey's bomb nor Sawyer fiddling with it should have let it actually detonate and kill any of the candidates.
One thing we do know is that Jin wasn't a candidate since he was effectively able to commit suicide. I guess it was Sun's name.
I actually think that the only two "firmly" established rules (and that is a rather loose "firmly" since we only seem to have some peoples word on it) is that:
A) Smokie can't kill any of the candidates himself.
B) People "touched" by Jacob can't kill themselves.
Remember, Richard isn't a candidate but he still couldn't kill himself, he wanted Jack to do it. Jack lit the dynamite, but stayed, so the dynamite fuse went out. Not killing Jack (and also, by proxy Richard). So being a candidate isn't the only way to be safe from suicide.
But apart from those two "rules" everything else is up for grabs.
So, there is a good chance that the bomb wouldn't have gone off, like the dynamite, if Sayid hadn't run away with it because Sawyer was standing right next to it and Sawyer was the one who triggered it (like Jack lighting the dynamite fuse).
But another thought to factor into those "rules" is about the "Island's" wishes. Apparently Michale wasn't able to kill himself either, because the Island "wasn't done with him". Ben also often mentions stuff about the "Island" wanting something, being done with someone.
So does the island have it's own rules and powers or is all of that really just an extension of Jacob's powers and abilities? When Ben talks about the "island being done with someone" is he unwittingly talking about Jacob being done with someone?
Did Jacob ever touch Michael? Is that why Michael couldn't kill himself or was it the Island?
If the Island also has powers/abilities over life and death then some of those rules above might be flexible. Is the Island's influence stronger then Jacobs? If Jacob touches someone so they will be safe from dying, but the Island wants them dead, what happens?
What causes someone to be crossed off the "list"? Sayid apparently was able to kill himself by running off with the bomb (unless since Sawyer triggered the bomb and Sawyer could kill Sayid, then jacob's power wouldn't protect him) does that mean Sayid was off the "list" after he died earlier and came back "wrong"?
I would really like to find out the "rules" of all of this who can live, die, commit suicide, etc... before the show ends, but I feel they may never go into enough detail to full answer al of the questions about it.