I think the changes in personality has been one of teh key factors of the show.
Locke went from being cool, to being a total nutjob cunt.
Sawyer went from selfish and cocky, to heroic and cocky.
Jack went from stoic leader, to paranoid drunk.
I'd disagree with you about Locke being a total nutjob. That's clearly him responding to the 'truth' about the Island and, in fact, he might actually be one of the sanest people on the show.
However, even if we accept that his characterization is that of "nutjob," it-along with the other personality changes you list-come across as organic and gradual responses to the time on the island and the discovery of what is really happening/has happened.
In Ben's case, however, it just seemed to switch overnight with no explanation. Especially since, unlike the others you mention, he has been aware of the truth about the island-and Widdemore-for nearly his whole life.
However, given the nature of the show and how time works on it, etc., I'm willing to keep an open mind about what's happening. And, in fact, as you note:
even when Ben shows sympathy, there is usually a reason for it, and its usually based on his manipulation of people.
So maybe this will make sense.
