Don knew that Charlie didn't always understand the way people thought.
Yeah, he could give you a run down on how people statistically thought but he didn't really get people. He didn't pay enough attention to them, what with being so caught up in his numbers most of the time. He thought in cold hard logic, and didn't pay any attention to the little feeling of dread right above his navel.
Don could deal with that because nine times out of ten, Charlie was right. But this wasn't one of those times. This was that one time Charlie couldn't predict how the bad guy was going to react.
The one and only time he got himself killed.