In Nella Larsen's novel about racial identity, Passing, there is no focus on discovering who is guilty of Clare's fall from the upper window. The idea of what happens is left up to the reader, so that conclusions can be made from the perspectives of the different characters.
It is highly unlikely that Clare's husband would want to do something like that to Clare because he was indifferent to her in many ways. It would hardly be a crime of passion, nor a crime of hatred: He simply does not pay enough attention to his wife to decide whether he loves her or not. Those who do not love, would not care to hate.
Clare enjoys life too much, especially its carnal pleasures, to end her life just for a moment of despair. As a woman who seeks pleasure in every way, Clare is more than able to surpass any heavy situation by switching to an alternative way to have a good time. Like, when her husband stops paying attention to her, she simply goes and finds another man.
Hence, the last alternative and the most feasible we have is that Irene either pushed Clare or caused the scenario for Clare to fall. We know this because the words that are used to describe Irene's feelings denote that Clare's death makes Irene feel liberated. She feels liberated from living under Clare's shadow, from feeling threatened by Clare's intense sexuality, and from fearing that Clare will take every bit of dignity away from Irene.
Therefore, the one person who mostly benefits from Clare's death is Irene. We can conclude that it is Irene who is responsible for Clare's fall.