Patriotism: Good. Nationalism: Bad. At least that is how it reads in my political lexicon.
Patriots love their country. They love the land and the sky, the people, the culture, the history and the values. If necessary, they are prepared to die for their country.
Nationalists feel all the above, but then take it a step too far. Sometimes several steps. And therein lies the problem. Nationalists (in my political lexicon) believe that their country is better than other countries. That it and their fellow citizens are superior to other countries and their citizens.
Sometimes that sense of superiority is applied not to national identities but to race or religion, such as White nationalists, Black nationalists or Islamic nationalists. But whichever vehicle they use, nationalists carry a strong sense of entitlement based on their nationality, colour or beliefs. And, if they are superior to others, than it must follow that whomever the others are, they must be inferior.
That is why nationalism is bad.
European colonialism was bad because it was at least partly based on the belief that Europe was bringing a superior civilisation to a barbarian world. Colonialism was a form of European-wide nationalism. In fact, much of the world that Europe viewed as barbaric had enjoyed the fruits of civilisation centuries before anything approaching civilised structures were even thought of in France, Britain or Spain.
Nazi Germany was the ultimate expression of nationalism. It encompassed race, language and culture, It claimed ultimate superiority for the German state and German race and used that assumed superiority to wage genocide and set out to subjugate the rest of the world.