"make someone's karma bad". Not really. You don't have access to anyone's karma, except yours. You cant make my Karma good/bad and I cant do anything with yours. Karma is created by the soul for itself. Karma is what you invest in the universe, be it thoughts, words or deeds, and how much you'll get back.
It's like the law three. Karma is basically reaping what you've sown.
If you've sown good, you will reap good
If you've sown bad, you will reap bad
So my suggestion to camryn is not to do a revenge spell because they can go wrong. Instead, relax with the knowledge that if this person has really done something terrible to you, he will pay for it without you having to try anything