That's a very interesting point... I don't think there are good guys in the show. Locke tries to be good, but fate keeps on pushing him to do immoral things (like killing that girl at the end of the third season). They are not essentially bad. They are just self-centered. For Locke to be a good guy, he should know what he's doing. They are all sincere (we can't deny that), but none of them are good... funny thing: the nicest one is, in a way, Sawyer: he would do anything for Kate to be ok, even sacrifice himself.
Regarding Widmore and Ben... Couldn't they both be the same? You are right: Widmore was very persuading when talking to Locke, but so was Ben in many occasions. Besides, the people hired by Widmore are far from being nice. Didn't he know what he was dealing with?
I know for a fact that the producers didn't consider at first a role like Ben's, but once the actor "did his thing", he became a regular in the show. (You can't just let an actor like that go). That's why I don't think at first they were considering the possibility of Ben playing a big part in the way the series ended, but now, I just don't know.