Yet also there encumbered sleepers groaned, Too fast in thought or death to be bestirred. (4-5)
What's keeping these soldiers in hell from getting a restful sleep? Maybe the guilt of having killed a bunch of people? Just a hunch.
The pity of war, the pity war distilled. (25)
This is Owen working a sentence like a boss. The "war is a pity" phrase sounds like something we might have heard before, but the second part of that line actually makes war responsible. Owen is placing the blame squarely on war for the suffering of these soldiers.
Now men will go content with what we spoiled. (26)
Poor speaker number two is racked with guilt. Not only does he feel bad about all of the fighting he did while alive, now he realizes that he's spoiled the world for generations to come. That's a heavy burden to carry.