you put parentheses around the "large" but I'm still gonna pick on this. For my part, I have yet to hear of a major jihadist attack in the west in which the perpetrators and their supporters were not completely surrounded by police and intelligence personel. In Germany we're watching a parliamentary commission pick apart what happened in the "lone wolf" case where someone drove a truck into a christmas market in Berlin. "You can't do anything about these things!" people exclaim. They're lone wolves after all. BS. That guy, and his supporters, were in constant contact with embedded sources around their milieu. I'm not a big friend of the police state as it is, and especially of intelligence services. And this story and others like it make it really hard to still believe in incompetence and bad coordination as the sources for all the fuck-ups that lead to him succeeding in the first place, and then the crucial witnesses being conveniently deported days after.
But my real point (sorry for digressing here for a bit) is that even in this in-depth investigation, the number of active supporters was tiny. And they were not even really organized. It was more like "I have a friend here ho will help me out, and one here, and one here." Your statement (even with parentheses) does not reflect how small these "parts" are.
I'm unconvinced; I believe it's not a tough sell. This is based on my observations of to whom people ascribe these crimes. They blame individuals first. Even if they should ascribe more blame to the organisations behind those individuals, those individuals are recognised (rightly or wrongly) as the primary culprits.
When I see news reports of such things, it's individual people that are presented as the culprits. News articles and Wiki articles name the individuals involved. While they do have organisations and groups behind them, if you ask people "who did this" you don't get a nebulous set of organisations; you get a name.
That depends on who those individuals are though. The media reports and reactions do change very much if you exchange some words, like replacing "islamist radical" with "white supremacist" or "christian fundamentalist" and "mosque" with "website".
You will certainly get individual names for each attack, the difference is whether having those names concludes the investigation or not. There are exceptions to this, of course, it's just my general expression.
That's a tough sell when (large) parts of communities are complicit, hiding, funding, supporting the individuals.
It's my impression that the primary difference is that we expect better from advanced nations and their citizens, not that there's a large difference in behavior. Denmark officially murdering people because of their sexual identity would be a shock. Saudi Arabia doing the same isn't, because we don't see SA anywhere near the level of (cultural, social, civilizational) development of Denmark. A child throwing a temper tantrum is normal, an adult doing the same raises suspicion of delayed development.