I have subsequently come across a few Imperial camps out in the sticks and, one way or another, killed everyone in the camp. Everyone except for the indestructible Legates.
Now, when I happen upon a decimated camp, the Legate is either studying the strategy map or sleeping in his tent or, in one case, patroling. At this point, don't you think they should be killable? I mean, let's face it: it is time to abandon post. Wars over, man. These whack jobs are still carrying on, looking for a weak spot in the Stormcloak defenses and the rest of the world has moved on.
These guys need help, or the sweet release of a battleax to the face.