The sentence indicates the act was motivated by revenge.
- “Vindictive” perfectly matches, signifying a strong desire for revenge. \(⇒\) Accurate.
- “Malicious” implies intent to harm, but not specifically for revenge.
- “Spiteful” is comparable, but less intense and formal than “vindictive.”
- “Callous” denotes emotional insensitivity, not revenge.
Therefore, the best word choice is vindictive.