Why does Eva set Plum on fire?

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.