Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
* In a sentence, "spitefully" would describe the manner in which an action is performed.
For example:
* "She glared at him spitefully." (modifies the verb "glared")
* "He spoke spitefully about his rivals." (modifies the verb "spoke")