What does it mean "to be evil" in an SEO context?

1. To copy someone else's unique content. That's plagiarism. No librarian is going to tolerate that. You have to write about your own ideas or at least write with your own words.
2. To try to cheat organic ranking by keyword stuffing. You can no longer "stuff" your keywords into content or website structures. You'll be docked in your ranking and perhaps even excommunicated from search engine rank entirely.
3. To cheat on the organic rank benefits of "backlinks" (by purchasing backlinks or creating "watered" down ones of your own). People should link to your content because you provided something interesting. For instance, I came across Ocean Feather's blog organically today after I entered a search term and their company blog post offered me the exact answer I was looking for. (That's how content marketing should work!)

Now, here I am, giving them SEO credibility by a) talking about them in positive light on my site and b) sharing their site with you via built-in hyperlinks. When search engines see a reputable site linking to Ocean Feather's content (i.e. a "backlink" to their site) the algorithms start to trust their content more. The Search Engine thinks, "Hey, someone good is linking to that site, it must be great content; I'll keep recommending it."

4. Also, posting the same article to a bunch of free sites simply for the inexpensive link back to your site used to be acceptable. But now algorithms see this as "a cheat" to strengthen your own position. (It's better if other people authentically recommend your content "because it's awesome", rather than you deeming it "cool" and re-submitting it over and over.)

5. And further more, you have to beware of your associations. You can't link up with people who do these "evil" things. That's what Ocean Feather means when they talk about creating "good neighbourhoods" online.