Vindictive indicates strong negative feelings toward another person. Her vindictive glare made it clear she hadn't forgiven him.
Pro Tip 2/3
Reaction to Harm
Use 'vindictive' when someone reacts with planned harm after being hurt first. He harbored a vindictive resentment towards his rival.
Pro Tip 3/3
Preceding Conflict
Vindictive behavior usually follows some conflict or harm done by the other party. Her vindictive actions were sparked by the unkind rumors spread about her.