The sniper scenario isn’t analogous (to me at least) because the war was over and (at any rate) he wasn’t in uniform. Outside of war scenarios I think it is entirely reasonable to consider an assassin who shoots someone in the back (and in the presence of his wife!) a despicable coward.
I fully agree that what Booth did was despicable. Given the extreme risk he willingly undertook, however, it doesn’t seem reasonable to me that it was cowardly. Shooting an unarmed man from the front would not have acquitted him of the charge of cowardice.
Americans have an odd relationship with cowardice/bravery. In its root meaning this refers to a person’s willingness, or lack thereof, to risk his own precious skin in the pursuit of a goal. It, of itself, has nothing to do with whether that goal or the methods used to achieve it are good or right or honorable.
The Waffen SS and the Japanese soldiery of WWII were, after all, incredibly brave and fought with as much courage as any group in history. The guys who drove planes into buildings on 9/11 weren’t cowards. Somebody who willingly detonates himself for a cause isn’t a chicken.
Which is not to say that all these men and groups didn’t do extreme evil. We have a, to my mind, peculiar tendency to denounce the evil of the cause (or its methods) and somehow morph that over into its supporters being cowardly.
Probably this is because we think of bravery as a virtue and cowardice as a vice, and we don’t like to assign any virtues at all to those we despise for the evil they do. The problem, of course, is that there simply is no rule that bravery will be limited to those who fight for a righteous cause, or for that matter that individuals on the side of right won’t be cowardly.
Bravery, like intelligence, competence and many other virtues, is value-free. It can be employed in the service of either good or evil.