It’s not about anyone being evil. It’s really not.
Patriotism and nationalism do overlap somewhat in meaning. In a venn diagram, they’d have part of their respective circles touching/overlapping. But they’re not the same thing. (They might have been when nationalism was first a word, as the article you site mentions @jaxk, but also as it explained, nationalism’s meaning shifted away from patriotism in a noticeable way.)
There’s a difference between “devotion to and vigorous support for one’s country” and “identification with one’s own nation and support for its interests, especially to the exclusion or detriment of the interests of other nations.” (Oxford Dictionaries).
There’s a difference between “love for or devotion to one’s country” and “loyalty and devotion to a nation, especially: a sense of national consciousness exalting one nation above all others and placing primary emphasis on promotion of its culture and interests as opposed to those of other nations or supranational groups” (Miriam-Webster Dictionary).
There’s a difference between “vigorous support for one’s country” and “patriotic feeling, principles, or efforts; an extreme form of this, especially marked by a feeling of superiority over other countries; advocacy of political independence for a particular country” (Google Dictionary). Note that those two bullet points in the Google Dictionary definition aren’t alternate definitions—if they were, they’d be numbered—they’re tie-ins to the initial definition.
Nationalism, as a word, carries this extra “thing” that patriotism doesn’t. It’s that “extra thing” that people are concerned about, not the part that overlaps.
If someone doesn’t know the difference between those two words, and thinks they both mean what patriotism means, that’s worth finding out to avoid miscommunication/misunderstanding. It’s probably also worth that person’s time to learn the difference between the two words so they don’t use one when they mean the other—don’t carry an “extra thing” into their sentence that they have no intention of carrying. Just generally, this sort of back-and-forth of figuring out what each other mean is a good practice for civil discourse (isn’t it?).
If someone doesn’t see the difference between those two things, doesn’t see how those two definitions are different (and how they lead to different outcomes), that’s worth finding out, too. But it would be a different conversation than the first. I hope it’s clear why. (If not, let’s have that conversation.)
And then there are people—a small number, but some of them have become rather well known at the national level recently—who treat patriotism and nationalism like they’re interchangeable on purpose. They want to blur the line and treat that “extra thing” like it’s an essential ingredient to plain ol’ patriotism. They want to get people to believe that “extra thing” is normal and healthy and maybe even necessary. And this, too, would lead to a different conversation than the first two.
I don’t know how it’s a personal attack toward someone for me to say, “Hey, when I hear someone use the word nationalism, I get concerned. That word evokes certain specific connotations for a lot of people, myself included. And it suggests certain attitudes and beliefs, certain ‘extra things,’ that I think are quite dangerous to a well-functioning, free, diverse society.”
When someone gets offended by that sort of conversation and walks away, I tend to wonder whether they didn’t want to talk about it because they in fact do believe those “extra things” that the word nationalism carries with it, but they don’t want to admit it. I can see from some of the reactions here that that’s not always the case. But then why else get so upset at the idea that a word carries connotations/nuance for people that you didn’t intend? Why treat that as a personal affront or a personal attack? What does that accomplish?