After Plum returns from WWI, he moves back into his mother's house. After a warm welcome, the family begins to notice that Plum is stealing from them, disappearing for days at a time, and spending his days sleeping. When Hannah finds the bent and blackened spoon from Plum's drug use, Eva realizes that her child who "floated in a constant swaddle of love and affection" no longer exists. Eva sees her act of burning Plum as "some kind of baptism, some kind of blessing" that will deliver her son from his addiction.
Eva chooses to kill her son Plum by setting him on fire. I think the reason she chooses this method is related to the universal symbolism of fire as cleansing. One of my students connected the burning of Plum to the mythology of the Phoenix rising from ashes.
I think that Eva sets plum on fire because she knows that he is not living his life right that all he is doing is waisting away so eventually he will end up killing him self if not by sucide or just life in general would have killed him because although he was a grown man he was childlike, not wanting to esacpe the comfort of his mother and his family to really wallow in his own pity of the man he had grown to become.