nationalism: patriotic feeling, principles or efforts. (The New Oxford Dictionary of English, Oxford 1998)
This is certainly not "evil". It is a most noble feeling which belongs to most human beings. People like George Washington, Simón Bolívar or Eamon De Valera had certainly this feeling and are now highly honored in their countries. And nationalism is not just something for statesmen, since almost all citizens of all countries in the world are prone to have this "patriotic feeling".
In spite of this, the word "nationalist" has acquired nowadays some rather strong derogatory meaning. In Spain, this word is applied to the people in Catalonia (and some other countries) who try hard to keep the own identity of our country against the status quo which makes Catalonia into a "province" of Spain. We are accused of nationalism as if it were a crime.
It has always been the case that the citizens of all nations which are strong and powerful are considering nationalism (i.e. not their own nationalism but the nationalism of the other countries) as a form of evil, something to get rid of.
The rather mild nationalism of the people in Catalonia is confronted all the time with a very strong and hostile Spanish nationalism or, to be more precise, imperialism (=a policy of extending a country's power and influence through colonization, use of military force, or other means. (The New Oxford Dictionary of English)).
It should be mention that the UN (resolution 1514 of 1960) has recognized self-determination (= the process by which a country determines its own statehood and forms its own allegiances and government (The New Oxford Dictionary of English)) as a fundamental right of the peoples of the World, while the Spanish Constitution of 1978 denies this right.