Eva's son Plum is the love of her life. As the only male child of a woman who loves men, he enjoys a certain indulgence from Eva that we don't see with her other kids. But war destroys him. He is barely recognizable when he gets home, and his mother and sisters quickly realize that he is a drug addict.
We don't get much insight into Plum's character, but he does tell us a lot about Eva. She loves her son so much that she can't bear the thought of him dying an unbefitting death, so she kills him. Plum is the one character who gets any real affection from Eva. She hugs and cradles him before she kills him. And although it might be hard for us to understand her actions, Plum's death shows us a part of Eva that remains hidden from everyone else.