Hope that there's no one out there who has vengeful feelings toward you, or wants to get back at you for something that you've done to them. A vengeful person is out for revenge.
You might have noticed that the words vengeful and revenge look somewhat similar. That's because their meanings are. The word vengeful is used to describe the feelings of revenge someone has toward another person or group who has done them wrong in the past. Think about Hamlet or Ben-Hur — those dudes were certainly vengeful.