I'm being sarcastic, the guy above you was a dumbass. Note the (/s).
Fighting effectively is the most ethical way to fight, you want the war to be ended decisively as soon as possible to lessen suffering. If that means killing sleeping soldiers, sure thing, the more of them you kill the less they'll want to fight and the easier they'll come to concessions.
It's not cowardly at all, just effective tactics, cowardice in war would be abandoning your post.
Don't forget you can do more damage with crippling shots than kill shots. The injured can be a massive drain psychologically and economically
Your comment immediately reminded me of this scene from Shooter. Start at :50 where The Band's great Levon Helm(playing an old firearms guru) explains it to Mark Wahlberg. https://www.youtube.com/watch?v=mn60YWO218k
(post is archived)