A reasonable point. But a sniper does not confront his target face to face. Are all snipers cowards?
BTW, I wish he had confronted Lincoln or his gun had misfired, as 30 years before Jackson’s attempted assassin’s did. Twice! Booth was 5’8” and slender. Lincoln, as old as he was, reportedly was still very strong. I suspect he would have kicked Booth’s ass.
Would you have considered the guy who tried to murder Hitler with a bomb hidden in the pillar of the beer cellar a coward?
I obviously don’t agree, but Booth considered Lincoln as evil as we consider Hitler to be.
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.