Just a quick note to say that it sounds like you might have missed a key element in the ending to this unusually profound story. This was one of the first stoies where Stan brought forth a part of Japanese custom which differed significantly from what western readers have been used to.

After Atsuo's mother announces that she killed her son, she says "Now, please, Usagi, slay me…."
"What mother could live after murdering the son she loves…?"

Usagi replys "I… I cannot…"

Obaasan comes back with "Please Usagi! My spirit is too weak to do it myself!" "Please…"

Skip some more dialogue from Obaasan until the final frame of her singing her lullaby to her dead son

"All has it season. Life is a cycle. It goes round and round. Round and round. Round and rou–"

That is when Usagi and Spot leave, heartbroken after Usagi has done what she had asked him to do…..