I'm not extremely knowledgeable on early communist China, so I'll focus on the USSR. Stalin made a concerted effort to eradicate the Russian Orthodox Church in Russia he went so far as to imprison priests, tear down churches, seminaries, and monasteries among other things. His hatred of religion mainly stemmed from the fact that his mother forced him into a seminary when he was younger, and Lenin's influence on him (and consequently Marxism's influence on Lenin). This attitude towards religion in the USSR carried on for a long time after Stalin died; as they did the same thing to Catholics in Eastern Europe in the 60's, 70's and 80's.
Basically I don't think Atheism itself was the main cause for the violence, but the various philosophies such as Marxism that call for a completely secular state caused violence for the same reason a Theocracy does. Both of those systems are set up in such a way that they can not tolerate religious diversity or else they would collapse. So yes Atheism can inspire people to violence; it's just done in a different way.
I don't think atheists are terrible people as a whole, but I certainly would argue that faith or the lack thereof does factor into how a person acts.