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}}