Why is tac nyan evil?

There's no evidence or information to suggest that "tac nyan" is evil. This phrase likely doesn't refer to anything specific or recognized in any cultural context.

It's important to remember that words and phrases can be used in harmful ways, especially when associating them with negative concepts like "evil." If you encountered this phrase in a specific context, please provide more information so I can understand the situation better.

It's always best to approach language with respect and avoid using terms that could be hurtful or misleading.