There are only three reasons you get violence on a demo.

One is when you have people trying to provoke counter-violence by the authorities, with the idea that this is a way of getting people more stirred up and "radicalised". And also that this is the only way there is going to be any coverage in the media (which is actually true often enough, but not useful coverage.)

The second is when there are agent-provocateurs deliberately trying to turn a peaceful protest into something which needs suppressing. (That doesn't necessarily have to mean official government sponsored agents provocateurs.) And their cover is likely to be to use the rationale in the previous paragraph.

The third is where there is heavy handed policing - and that happens often enough, and not just in dictatorships. When it does there are normally likely to be some people who react violently. (And this can then get adjusted by the media. There was a classic case in the TV coverage of an episode in the British Miners Strike in the 80s - newsreels showed miners throwing coins and stones, and then the police charging on horses. In fact the vide had been doctored - the police charge came first, and the miners were in fact throwing things to try and drive off the horses.)