why does the older dead advised the newer dead in these word ;let us sleep now

The speaker tells the soldier that there is no reason to mourn, and he replies that there is –it is the "undone years" and the "hopelessness". The soldier says his hope is the same as the speaker's; he also tells him he once went hunting for beauty in the world, but that beauty made a mockery of time. He knows the truth of what he did, which is "the pity of war, the pity war distilled", but now he can never share it.

The soldier/vision continues, saying men will go on with what is left to them, or they will die as well. They will not break their ranks even though "nations trek from progress". He used to have courage and wisdom. He would wash the blood from the wheels of chariots. He wanted to pour his spirit out, but not in war.

Finally, he says to the speaker that "I am the enemy you killed, my friend," and that he knew him in the dark. It was yesterday that the speaker "jabbed and killed" him, and now it is time to sleep.