The violent rebellion and bloodshed finally reach their inevitable conclusion through divine justice. God ensures that everyone involved faces a punishment that exactly matches their actions. The harm caused by the men of Shechem is brought right back upon their own heads. This brings a prior warning to life, as the curse spoken by Jotham in his parable of the bramble is completely fulfilled.
In Jotham's story, Abimelech was represented by a bramble. Just as the parable warned, a destructive fire spreads from this bramble to burn down the trees. These trees represent the men of Shechem, who first crowned Abimelech as their king and later turned against him. Ultimately, this same destructive fire consumes Abimelech himself, ensuring that he too receives his exact punishment from God [ביאור שטיינזלץ].