Likely yes, they are. There were people murdering Jews well before Internet, TV, radio, printing and arguably even written language existed. There still are. Attributing this to a random wacko, without specific casual proof, is likely to be wrong. Of course, in some cases (yes,
that case, thankyouverymuch) there's ample proof. In most cases we're dealing with on the Internet, there isn't. And a single reference by a demented individual, without further proof, can't be taken as such. Otherwise you basically pass the control of the moral norms into the hand of demented wackos - the only thing they need to do to ruin anybody is to commit some atrocity and declare they were doing it in your name. Obviously that alone can't be a proof of anything. You need to examine whether they were sincerely inspired by those ideas, where those ideas indeed intend to inspire such reaction (some people are sure TV is full of secret messages telling them to kill the President, that doesn't mean we have to shut down the TV broadcasts) and whether this reaction is something that is inherent in those ideas.
Also, there is a myriad of possibilities between "totally unrelated" and "have direct causal link which imparts responsibility". If somebody is on the left politically, is he "totally unrelated" to Lenin and Pol Pot? Not exactly, they probably share some ideas somewhere. Is he responsible for all atrocities Lenin and Pol Pot committed? I don't think many people on the left would agree to that. And here we have direct link between ideology and actions, acknowledged by everybody. You are extending it to actions of insane people which may be just using ideology to dress up their demented impulses - this is even worse distortion.
People are responsible for their actions. But if some crazy person mentions somebody in a positive sense, that by itself does not impart responsibility on the person they mention.