Is it?

http://freespeechdebate.com/en/case/the-brandenburg-test-for-incitement-to-violence/

Brandenburg was found guilty of violating Ohio state law, which prohibited “advocat[ing] . . . the duty, necessity, or propriety of crime, sabotage, violence, or unlawful methods of terrorism as a means of accomplishing industrial or political reform,” as well as “voluntarily assembl[ing] with any society, group or assemblage of persons formed to teach or advocate the doctrines of criminal syndicalism.” His penalties included a $1,000 fine and a 1-10 year prison sentence.

In a landmark judgment, the U.S. Supreme Court overturned the conviction, contending that the Ohio law affronted Brandenburg’s freedom of speech, protected by the First Amendment of the U.S. Constitution. Instead, the Court held: “Freedoms of speech and press do not permit a State to forbid advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.” Because the rally was not obviously intended to incite specific acts of violence, and because it was not likely to do so, government restriction of Brandenburg’s speech was unconstitutional.

I'm not from the US, I was under the impression that general incitement to violence was legal "_____ are terrible people and should be killed!".

The only area I'm aware of in US law where speech is limited criminally has to do with threatening someone.