It is morally wrong to cause pain to another person. Therefore, dentists are immoral people.

You ever have an abscessed tooth THAT my friend is pain. The dentist is preventing an abscess from ultimately happening. So no dentist are not immoral.

That's the "Sweeping Generalization" fallacy.

I think it is actually a Faulty Generalization, specifically a False Analogy.

You are arguing an analogy between a dentist performing helpful and necessary medical procedures which can be painful in the short term to physical injury caused by another without medical or positive effect, only to designed to cause harm.