It is no fun hanging out with vindictive people, who are forever out to get back at people they think have hurt them. If you forget to say hello to them one day in the hall, they will carry a grudge against you into next week.
Vindictive is often paired with mean, as in "the atmosphere of the cheerleading squad at my new school was vindictive and mean and I wanted nothing to do with it." Vindictive rumors show a spirit full of revenge. Vindictive is from Latin vindicta "revenge." The related Latin verb vindicare has the very different meaning "to defend or clear someone from guilt," and this is the source of the English verb vindicate.