The Trump campaign and its collaborators have mobilized this sense of grievance against the other in strategic ways. So have far-right assholes in Europe and elsewhere. They do it because it works.
In the case of both ISIL and the Trumpists, it works because of 1) a partial truth in the grievance, and 2) a kind of moral hypocrisy or lack of alternative on the "other side" to the side presenting the grievance.
In the case of Trump, present-day mainstream "Liberalism" is a hollow shell, divorced from either it's previous image as an occasional advocate for the working class, or civil society more generally, both of which have been neutered by neoliberalism and it's slow transfer of all civic values into economic ones. Witness the number of liberals these days who have positive associations with giant monopolies like Google and Facebook, simply due to one-dimensional perception of their approach to issues like "diversity"...whatever "liberalism" once was, it isn't that anymore. It's pitiful these days, listening to some liberals talk, it no longer surprises me at all that Trump made it in. I wish it did, but it doesn't.
In that vacuum, Trump can play off legitimate anxieties. Will he fix any of it? Oh of course not, he's not capable, nor does he want to. The thing is though, in a strange way some white people ARE discriminated against, both culturally and economically. It's not actually because they're white people, it's because they're workers and rural poor people..but in the present environment (just think, it is still ok to mock "dumb redneck white people" in a way that would be completely unthinkable with other groups in liberal circles) I can see why they mistakenly think it's about their race and culture, rather than what it's really about - their class and place of economic disadvantage. Hell, Trump's rhetoric even freaking plays to that, once the "deplorables" thing happened, the rest wasn't that shocking, at least in retrospect. it's easy to hate Trump (really easy, he's disgusting and hilariously clueless on so many levels) , but not to look at the sucking vacuum that allowed him to be. When was the last time the mainstream Democratic presidential candidate talked that much about infrastructure, the negative effects of trade deals, etc.? Obama sold his shit out almost right away.
ISIS grievances work much the same way. The US doesn't kill Muslims because they are Muslims, but the fact that we have so many years of regime change, military occupation, drone bombings, and double standards about human rights in Muslim countries makes it quite easy for them to paint that picture. It's not a narrative that makes sense upon examination, but they are counting on the emotional content of the grievance overriding that, and it does.
In short, the only real "fix" is a more attractive, less crazy alternative..in the case of Trump a political movement that can actually address the legitimate economic disenfranchisement (including disenfranchised whites, believe me, they exist, I see it working in addictions) he leverages without all the idiotic racism, hypernationalism, and the other drivel that comes out of his "platform"..there's a reason populist rhetoric works. On the side of ISIL the only fix I can imagine is a movement that legitimately cares about the self determination of average Muslims and/or Middle Easterners, allows it, and presents it as alternative to the insane violent feudalism. The issue there is enough years of crappy theocracy (and lack of autonomy due to interference form the US, etc.) to stand in the way of something like that even coming into being.
Short of those things existing, I personally find it really unsurprising that either one is successful in hawking their grievances.
