Wow! This is a great and deep question!
Ethan Frome by Edith Wharton is multidimensional in the way through which it expresses the basics of human and universal compassion.
We first experiece the compassion as readers when we learn about Ethan's current situation: he is poor, unhappily married, dissatisfied with life and with no future hopes. Yet, he yields to the minimalistic aspects of what could be deemed as joy, namely, Mattie.
Second, we get to know the oppression with which he lives under his own roof. His wife, Zeena, is passive-aggressive hypochondriac who manages his every move. However, Zeena, his also compassionate. She took care of Ethan's mother when she was ill. This was ultimately the reason why Ethan married her. Now it is Zeena who thinks she is ill, and crushing the marital budget with unnecessary treatments.
Then, there is Ethan's love interest, Mattie. She, herself, is a subject of compassion. Young, poor, and destitute, Mattie has to live under the Fromes's roof as a maid under the regime of Zeena. To make the story even sadder, both she and Ethan fall in love. That is worthy enough of compassion, if not pity.
Finally, the climax comes when Ethan and Mattie have had enough and refuse to be separated by Zeena. So they quickly make a (very dumb) suicide pact of running towards a tree on a sleigh...and then the worst part happens: They both survive and become crippled. And, who is their caretaker in the end? Zeena.
After all the nightmare is seemingly over, we get to experience from the viewpoint of one narrator who is telling the story from his personal meeting with Ethan Frome. And that's when we realize that the entire town had felt the pain of Ethan and Mattie. We also realize the contempt of the town towards Zeena. Yet, how could we accuse her of anything when she is demonstrating compassion by serving a husband and potential mistress who nearly completed a suicide pact to get rid of HER?