Hey, remember the Crusades? This is not unique to Islam. Any religion can be made into a tool of peace or a tool of war. Wicked leaders will interpret their sacred texts however needed to further their political goals.
"Violent white supremacism is a symptom of multiethnic societies."
All modern societies are multiethnic societies. Once white supremacy is done with the obviously foreign-looking people, it will start to eliminate those who are not white enough, resulting in an existential and violent battle over determination of the "volk". As a descendant of white immigrants whose ancestors would not have been considered fully white in my country, I would not want to live in this environment. No doubt that I would be on the chopping block at some point for not sharing x% blood with some arbitrary progenitor.
"Overall the solution is simple: muslims can have their own countries where they can do what they want, and other people can have their own countries with their own rules."
Well, as a white person in a historically white country, I say Muslims are welcome to be my neighbors, and anyone who disagrees can go live somewhere else.