RE: Is Nationalism to blame? (It sounds like you mean Imperialism)
I oppose nationalism because it tells people that person X is better than person Y because X was born in country Z. (which I find unmoral)
No it doesn't.
It simply says to have pride in your nation. To take care of your nation. Yet to have pride you also need to have a reason for such.
Can that be weaponized by propaganda? Sure. Yet the same can be said of every Socialist and Communist attempt as well.
In fact the boogeymen people like to hold up to point out the woes of Nationalism people like to conveniently leave out that those places also considered themselves a socialist movement. Yet ultimately because they were focused on expanding and controlling outside of their own borders they were not Nationalists. They were imperialists. Yet back then imperialism was viewed negatively, so they are not going to come out claiming to be imperialists.
I think pride here is the important point.
Pride never seems to stop at the (positive) nationalism and always go towards the negative nationalism Chauvinism (which often turned into e.g. Hitler)
It is the difference between inclusive and exclusive nationalism.
For example you could say I am an inclusive nationalist. I am a fan of our Grundgesetz (constitution) and want as many people as possible to have it's protections. Immigration? Everyone's right. Everyone can be German, if they want.
But do I feel pride? No. Why? Putting aside that a lot is still bad, how can you feel pride towards something like a state?? I might feel pride that we uphold our constitution standards, but in the thing itself? I did not wrote a single word of it. I did not build a single meter of Autobahn. How could I be proud of it?
Than there are the "rights", the exclusive nationalists. They define Germany first point as a difference to others. You can't just come here, and becoming German? No way! This is our country, our Autobahn, our language (which is always one the biggest jokes).