It's disturbing to see it happen--I've watched the videos, too--but it's happened before, and been a lot worse. Like late in George Wallace's campaign in 1968, when he'd attract as many protesters as followers (course, we're still in early-stage Trump, there's time yet).
Actually, I could only locate one news account of violence at a George Wallace rally. Odd, it's one of those suspicious things where I remember Wallace rallies as violent, but maybe I'm influenced by the violence at Trump rallies? No YouTube archival footage, either. All I found was this trip into the microfilm archives from October 1968, in Detroit:[NB: It's an iframe of a picture of a newspaper article and isn't displaying. Try http://archives.chicagotribune.com/1968/10/30/page/8/article/wallace-clips-detriot-speech-short-as-violence-breaks-out
http://archives.chicagotribune.com/1968/10/30/page/8/article/wallace-clips-detriot-speech-short-as-violence-breaks-out
Sounds like a full-on riot, and Trump's not there, yet.
After a little more searching, and watching videos of Wallace rallies, it looks like his followers in 1968 were actually rather polite for the most part, sitting in their chairs and clapping and hooting almost decorously. Things got a little more violent in 1972, and that's the campaign in which Wallace was shot. It wasn't until the 1976 campaign that the crowd surpassed (current) Trumpian levels of hooliganism:
...at a Wallace for president rally in downtown Boston. In 1975, that city was contorted by its own race war over school busing, and the enormous two-tier assembly hall was packed. It was an angry crowd — a black television cameraman was punched as he walked up the aisle. In the middle of Wallace’s remarks, there was a loud explosion, and Wallace, who had been paralyzed by a bullet three years earlier, fell forward from his wheelchair into safety behind the podium.
The noise was caused by a crashing klieg light, knocked over in a fracas as a heckler in the balcony was attacked by the crowd. As Wallace clambered back into his chair, his supporters beat the protester bloody and tried to dump him over the balcony rail.
Source: NY Times, 2008: The George Wallace We Forgot
"...his supporters beat the protester bloody...", well, so far the security guards at Trump rallies have managed to rescue the protesters before things got that bad, but from watching the 2015 videos, it's clear that it was only intervention by security that saved those people. I mean, "light him on fire"? Holy crap, that's a lynching reference.
What can I conclude from all this? Trump hasn't achieved Wallace levels of violence yet, but the trends all point that way.