Ethan Frome is concerned with personal morality and puts forth a number of complicated moral quandaries. For example, should Ethan Frome pursue true love and run away with Mattie, or should he stick it out with his wife Zeena? The characters face numerous conflicts which pit their personal desires against their perceived moral and ethical responsibilities.
Because Ethan loved Mattie and didn't love Zeena, it would have been morally correct for him to run away with Mattie.
In Ethan's moral code, it's more important not to hurt people that it is to find personal happiness.