Except for the fact that if you really look at history, and not just as an excuse for spewing political stereotypes, you'll see that actually most actual revolutions weren't particularly Marxist at all. Most actually followed the same pattern of lower class revolts that happened since long before Marx was even born. In fact, you can see the same pattern in the servile wars in ancient Italy, and that's one thing nobody would blame on Marxism. When the conditions for the poor get too unbearable and for too long, they'll pick up their pitchforks and torches and follow some guy who promises them something better to the castle gates.
(And then occasionally it turns out that he doesn't know what to do once he's beaten the tyrants' armies, except prove Peter's Principle. Someone who got promoted to leader by just being willing to kill whoever stands in his way, unsurprisingly might only be good at killing more people and/or want to kill more people. And if you thought I'm talking about just the likes of Lenin and Mao, try Spartacus too. And that one wasn't a Marxist.)
1. Seeing that as just Marxists being violent is just silly. I can see why people would prefer to see it as that, rather than face reality though, and, again, it's a pattern we see all through history. None of the nobles wanted to think "man, maybe we've been pushing those peasants to the limit", so they invented some BS explanation instead.
In reality the whole story is that of a breaking point. There's only so far you can push people towards bare subsistence and even below, so you can get a bigger castle, before they show up with torches and pitchforks in front of said castle. If you think it's Marxism that made them violent when that happened in Russia, China or Cuba, then you're just missing the point.
2. There's a reason why they added Lenin to Marx and Engels in the USSR. Because the idea that you can have just a vanguard of True Scotsmen... err... True Communists and just shoot whoever didn't get the class consciousness yet, was actually not very Marxist in the first place.
They had to actually rationalize a revolution that wasn't really much like what Marx envisioned, and in a country that wasn't quite at the point where he envisioned that happening either.
Which again should be enough clue that seeing it as the Marxists being violent is pretty much believing that the tail wagged the dog. What they actually had was more like pushing enough people to the point where they wanted to revolt, and then a bunch of them had to figure out how the heck to fit Marx in there.