To want revenge is normal. Even if there is another dimension, such as sadism, it's still not 'a symptom of mental illness'. Is anybody actually telling you that you are mentally ill? The point is simply that telling people that they are mentally ill can be a form of abuse.
I believe it is normal to want to take revenge in various ways.
When there are reports on the news regarding cruelty, many
compassionate people feel the urge to exact sadist revenge.
However, there is such a thing as Sadistic Personality Disorder, so
it might be worth taking a look at the symptoms of this and seeing
if you have anything in common.
I believe indulging in these fantasies serves as a temporary relief, but I imagine it can have detrimental effects to you if they prevent you from forgiving or moving on from the past. Clearly it has made you question your own mental health, so regardless of whether or not you can be termed as mentally ill, I think it is worth taking steps to get you away from these fantasies altogether so they cannot lead to this.