It is "Us" vs "Them" which makes nationalism evil
What is the necessary condition if I want to hurt another human being? I need to lower my appreciation for this person. So I begin to compare him with things I find worthless and disgusting: animals, excrements etc. etc.
This is a foolproof way in history to suppress remorse and stop thinking about what I am doing to another human.
Now we all know that there are differences in humans and associated groups. We all have a history, a specific environment, genetic differences, rules and cultures which not only make human unique, but bigger groups which share common traits. That alone is not a problem.
The problem begins when we start to give us ourselves a higher status for things which don't deserve merit. I am working harder, so I earn more money. I study hard and get good notes. I am doing many things for others, so others give me more responsibility (higher rank).
I am also born in Germany which makes me a superior person. Isn't it fantastic that I needed to do absolutely nothing to be better than other humans apart from other Germans? Birth, plop, and I rock. Uh, you are asking why do we Germans rock? Oh, it's easy, other Germans have done great things. Uh, not me...personally, but I and other Germans are part of Germany so we deserve a part of the merit.
What I describe above is nationalism.
It is a very thin difference between acknowledging and appreciating a culture and starting to feel ourselves special for being a part of the culture. Once we go that way we are doing what Dietrich Boenhoeffer called "cheap grace": Gaining merit without acknowledging responsibility or cause. Yesterday I injured someone badly with the car and drove away. But I go to the church, buy a candle and eat a wafer and I am absolved.
Nationalism fuels also emotions and make people reckless, careless and immunized against criticism. Someone is claiming country X is doing evil things? Then it must be "anti-X-ism". Someone who cannot stand that people in charge are doing things which are evil and harmful and speaks finally about it? Then it is a "traitor". People are also craving for recognition and if you give them some form of identification which makes them feel important, they are afraid that someone is taking it away from them. This makes it often impossible to discuss things out and is a cause of the backfire effect: Confronting someone with facts which may threaten a worldview often cause that the belief strengthens. It must be right because then I am not wrong.
Nationalism hides also responsibility. If you think over it: There is no such thing as the will of a motherland. Does the ground on which you are standing suddenly forms a mouths and says to you: "Hello, I am country X and my wish is that you defend me" ? No (at least I hope you do not experience it). So at the core what exactly do we mean if we say: "It is the wish of our nation..."? It is the wish of your leading figures, nothing more. But you can disagree with a leading figure because you know (s)he is a human and can make severe errors. The trick is if you use a "nation" you don't know how to articulate criticism and who should be addressed.
It should be said that there is in fact obligations between humans and groups and if you violate contracts, the results are every bit as real as physical items. The problem is that leaders hide the result of their actions under nationalism which is the reason why it is so damn hot for all dictatorships.
What most people also don't realize that history does not support the idea that when conflicting viewpoints meet that the "better" one will win. It is absolutely possible that if the USA lost the war, George Washington would be known as despicable traitor of the British Crown and Benedict Arnold gets a 20 m marble statue after recognizing that he fought for the wrong side.
Ever wanted to know what people say if they are responsible for mass killings? "But I am a nice person". Nationalism is one possible step to divide people in lesser and higher beings which makes it easy to walk in the wrong direction. This can be generalized for nearly every form of -ism: Once a specific form of superiority apart from any personal merits is established, all their negative effects start to creep in. Double standard (My evil is necessary and comes from good will, yours is pure malice). Discrimination. Groupthinking. Immunization against criticism (Naturally I should not talk with you because you are X!).
And you can get even more bang for the buck if you are even more special. I am not only German, but white. Wait, you need to be white to be a genuine German! So once you have started with one -ism, it is like a supermarket. More self pimping without any repercussions. This is the reason racism is often associated with nationalism, you are starting to build specific images in your mind how the orderly world should look.
ADDITION: There is another reason nationalism goes very often with racism/culturalism. The problem is: Everyone is able to acquire a nationality because it is an abstract concept. The abstract concept nationality is often, but not always linked to a specific culture (African countries are a notable exception: Their borders were literally drawn by a ruler and their respective governments have also often zero direct influence to the tribes they are purportedly governing). Now every country is principally able to naturalize a new citizen at once (give a passport and updating documents) so native inhabitants often feel that a former alien is "cheating" by acquiring nationality without visible effort. While this position may have merit if the immigrant actively ignores or even rejects the culture (s)he is joining, it falls apart once the immigrant consciously decide to be a part of it by learning the language, culture and customs. Such people are now posing a problem. Either we must accept that having a specific nationality has no merit in itself (I am not talking about privileges of a nationality like social security, less limits to travel etc, I am talking about the privilege of being nationality X itself) and accept that those newcomers have every bit to call themselves to be a member of nation X or we deny them that right because those people are "devaluing" the "merit" of being a specific nationality and invent arbitrary divisions to associate a nationality with a culture/race suspiciously close to the own ideas and hold the exact borders as hazy as possible to include/exclude specific persons.
Some answers illustrate exactly the very specific dangers of nationalism.
Furthermore, the constitution and political culture recognizes the rights and freedoms of people regardless of racial origins. Consequently, the erosion of freedoms and rights of any person based on race, rather than merit, is considered contrary to American values and therefore 'bad'.
Nope. Bullshit. As every black US citizen can testify that the constitition was in effect since a very long time and did not prevent "Jim Crow laws". You know, the constituition was only for..."real" people. Not dumb..primitives or women who cannot think. And there was no change in German laws for murder during the Nazi era. It did not protect Jews because...Jews are inherently evil that they do not deserve protection. Slavs were also predestined to be subjugated, so if one dies, who cares?
Nationalism is so dangerous because the content is malleable and can be targeted against specific subgroups. As I said, once you accept different treatment for "good" and "bad" people, the result is not pretty.
I really don't give a damn if anyone claims specific "values" if these "values" change with time, and surprise, changes in the direction of the leaders.
Well, the part with the Slavs during the Nazi era did not work out. What happens when the illusion disappears? One German soldier described in an article what happened. He was taught about the inferior Russian soldiers. After a fight which forced a retreat he hid in a cellar of an abandoned building. Suddenly a submachine gun appeared in the broken window and a voice in broken German said: "Get out, Fritz!". He got out with shaking knees and saw three Russians, none of them older than 20. And they got him. He was at their mercy. He recounted that he never felt such a strong shame and embarassment before in his whole life.
Other conversions occur once the person comes in contact with defamed X and starts to see that they are the very same stuff: No villains, but also no angels. Good and bad people, but mostly the average person: A bit nice, but also driving fast, smoking weed or evading tax.