The sentence highlights that the act was one of revenge.
- “Vindictive” means having a strong desire for revenge. \(\Rightarrow\) Fits perfectly.
- “Malicious” means intending to harm, but not necessarily for revenge.
- “Spiteful” is similar, but weaker and less formal than “vindictive.”
- “Callous” means emotionally insensitive, which doesn’t imply revenge.
Thus, the most appropriate word is vindictive.
\boxed{\text{Answer: Vindictive}}