What did Charles Widmore mean when he said he knew WHAT Ben was? Is he implying that Ben is not human?

Chuck could have been implying Ben was a lousy human being, just as Ben succinctly described Keamy. Afterall, Ben was responsible for The Purge. I would think there's more to Ben than meets the eye though. He's clearly special.

I agree with much of what rabidranger said, but I also believe that when Charles said "I know WHAT you are," he meant that he knew that Ben is a cold-blooded killer. Someone who is willing to do whatever it takes to "win."

I know, ryan, that comment made me react the same way at first - as if Ben must be a clone or alien or some other 'thing'. I don't believe that's what Widmore meant though.

I agree with the comments made so far, that Widmore knows how cold-blooded Linus is.

But, it was a different comment and delivered in a different way, than when Miles said 'I know who you are and what you can do'.

Some rambling: Blade Runner parallels? Ben can't kill him because it's "programmed" into him, but Widmore can't kill Ben because Ben knows where the island is. Ben is one of a number of genetic experiments designed to create people who can live on the island without going crazy or who can serve as "constants" or conduits to the island experience (this is why the kids are so important) or who can travel in time at will. Ben sure is good at killing people, thinking ahead of them--as if he's not quite human. And for all of his crazy ability, Ben needs Jacob to direct him. Walt is another experiment, as is Aaron. I think we have hints that Jack and/or Claire were experiments (being involved drives the parents to self-destruction).
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Let's say Hanso and Widmore started out working together but split at some point over how the island should be used. Ben remains loyal to Hanso (who keeps communicating through Jacob) while Widmore wages the war.

People talk like Widmore did when they want to dehumanize you. It was just Widmore looking down his nose at Ben. But we've seen Widmore be incredibly rude before.

And have you noticed that Ben is always polite when he speaks? But then, Ben is the master manipulator. Widmore clearly believes he has enough power that he can get away with being rude, even to someone who is going to try to hunt down and kill your only daughter.