All the times Desmond saw Charlie die he didn't think about just "letting" it happen.
Like when he saw Charlie drown he saved him.
And when he saw him get struck by lightning he saved him.
Then he thought he would benifit from Charlies death.
When Desmond had the vision of what he thought to be Penney comming to the island, he was going to sacrifice Charlie. When the time came to do that he changed his mind and pushed him out of the way of a flying Arrow.
He went to Charlie and told him about Claire and Aaron getting into a helicopter in his vision, trying to convince him to die!
Why would Desmond want
Charlie to die if he would not benifit from what he saw in his Vission?
I think cause Desmond lied about what he saw in his vision!
The only other time he wanted
charlie to die was when he thought he would be with Penney.
So heres what Desmond saw: Room with equiptment, Yellow Blinking Light, Charlie turning off the light, Charlie Drowning, A boat with Penny on it. (Season 4 Finally)
The only way charlie would be OK with drowning is if he was told he could save Claire and Aaron.
Desmond paddled out to the looking glass with intentions of letting charlie die, but he changed his mind again and was going to take Charlie's place.
Both times he felt bad for lieing to Charlie. But Charlie, being the great hero he is, wanted to "save Claire."