Are you alowed to insult a person, yes absolutely, but it only proves you’re an absolute asshole. ;)
You can even state you think an ethnic group is bad. You can’t state the same ethnic group is trash and should be whatever. That’s discrimination and not allowed in democratic or even civilised societies.
In democratic/liberal societies you solve your differences via debate. The result may even be that the parties don’t like each other, but as long as they can decide to live in peace together it’s alright. Agree to disagree and continue to live your life is fine, trying to harm others isn’t.