Symbolism, Imagery, Allegory
Purifying Flames
Yeah, when two of the prominent characters in a novel die fiery deaths, you'd better believe that "fire" is going to wield some symbolic heft.
Fire appears throughout the novel and results in the deaths of both Hannah and Plum. Both of these deaths are tied up with the character of Eva: she's directly responsible for Plum's death-by-fire and she tries to stop Hannah's burning. But this connection to fiery mayhem isn't as damning as you might first believe—these flames are seen as somehow cleansing.
When Eva douses Plum in kerosene, he feels like he's undergoing:
Some kind of baptism, some kind of blessing. (1921.49)
Remember, Eva kills Plum in part out of mercy: Plum has come back from the war so changed and distressed that she kills him so he dies while he's still "a man."
And when Hannah catches fire, Eva's motherly instinct prompts her to throw herself out of a second-story window in the attempt to save her daughter. She's not successful, but the act shows her selflessness as a mother.