Lines 40-44 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I am the enemy you killed, my friend.

Hold up. Speaker number one killed speaker number two? Yikes. They must have been enemies on the battlefield.
You've got to give Owen props for the cleverness of using "enemy" and "friend" in the same sentence. Things are clearly different after death. And you've got to give him points for style, too. Way to hold on to the key information until it'll have the most impact. Wilfred Owen: dropping truth bombs.
This explains the "recognition" in his eyes in line 7. If he killed him, why does he address him as, "my friend"? Well if you were paying attention to anything the guy was just ranting about for like thirty plus lines, you'd know that he thinks war is a bunch of bologna. It's the worst thing ever. Maybe he doesn't distinguish men as enemies or allies anymore, but all fellow men damned to spend the rest of eternity in hell. In a sick way, they're in this together.

I knew you in this dark; for so you frowned
Yesterday through me as you jabbed and killed.

These lines give us a little chill. Speaker two recognizes speaker one because when he was stabbing him to death he was frowning in the same way he was when he just encountered him today.
Owen writes that he frowned through the man he was killing, which is really interesting because it's almost as if he can't see him, or like he's seeing through him. Because speaker number one saw speaker number two as an enemy and nothing else, he wasn't able to recognize him as a person, and couldn't truly see his face. And now that he has to "face" his enemy in death, he sees that they're so much alike. Owen is getting deep, y'all.

I parried; but my hands were loath and cold.
Let us sleep now. . . ."

Vocab alert: parry means to try to block, and loath means reluctant or unwilling.
So speaker number two says he tried to block speaker number one from killing him, but his hands were unwilling—maybe he was tired of fighting and wanted to die in some way. Or at least to give up trying to survive.
This image of speaker number two with his hands raised is actually the second time it's appeared in the poem. The first was when they first meet in hell, "Lifting distressful hands as if to bless." Owen seems to be getting across that even though speaker one killed him, speaker two has forgiven him.
The final line seems like it should be the beginning of peace for these two soldiers, but we've got to remember that they're in hell. When speaker number one first arrived on the scene the "sleepers" were all groaning in agony. You get the sense that even if they sleep, they're still going to suffer. The ellipses at the end of the final line makes you think of that, too. He's trailing off from his rant, but it's hard to believe that he's found peace.
This could definitely go down as one of the bleakest war poems in history. This is not one you're going to want to bust out on Valentine's Day, folks. The forecast for this one is dark and dreary with a 100% chance of hell.