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In the comments multiple people are making the mistake of thinking that nationalism is the belief that nations should exist.
That's not nationalism.
People can be against nationalism and still believe that nations should exist. Gandhi was just such a person, and he was murdered by a Hindu nationalist.
Nationalism is the belief that the group you're part of (usually tied to some national identity myth) is superior to others, and usually goes hand-in-hand with xenophobia, racism, bigotry, and fascism.
From the article:
""Godse did the right thing by killing Gandhi," said Devendra Pandey, 53, national secretary of Hindu Mahasabha, the Hindu nationalist organisation behind the memorial library. He was vocal in his belief that India should be declared "a country rightfully for Hindus" and that its 200 million Muslims should move to Pakistan."
Those 200 million Muslims who live in India and are part of the Indian nation, yet they're not Indian enough for the nationalists, just like German Jews were not German enough for the Nazis.
There are close parallels in other nationalist movements from all over the globe, in calls for "America for the Americans" where you have to "love it or leave it" and calls for English to be made the national language and Christianity the national religion... very much like Hindu nationalists in India calling for Hinduism (their own version) to be the national religion to the exclusion of Muslims and other minorities.
Violent repression of people outside the in-group is another feature of nationalism, and again it has nothing to do with believing that nations should exist, but has everything to do with feeling superior to the people being murdered, tortured, robbed, discriminated against and driven out of their homes.
This is why Neo-Nazis are "white nationalists", and you're more than a little confused if you think they are some sorts of defenders of the right of nations to exist and not bigoted, hateful, violent oppressors of everyone not exactly like them.
This is why nationalism is pernicious, whatever the flavor.