I doubt it’s that one dimensional. One could point out that making things equal results in (edit) even more terror (as most former communists countries prove). Of course that reasoning would ignore the Scandinavian “exception“.
We're socially wired to keep things "fair", and when very unfair things happen, we have an almost religious feeling of righteous indignation. If someone is cheating society, we want to punish them and set things right. This is why there's a tendency towards lynch mobs, or their modern twitter equivalents.
Equivalently, when someone feels excluded from the basic things he sees others enjoying freely (food, sex, shelter, leisure), he also feels this injustice, and will want to lash out, punish, and bring justice back to his society. If he's excluded for long enough, or to an extreme enough degree, he'll stop associating himself with that society, and possibly even consider replacing it with a better one. Of course, not everyone who feels the bad end of inequality is going to take up a gun and start shooting, but the less emotionally stable you are from your circumstances, the greater the risk. These "manifests" are a last-ditch effort to be listened to, respected, and included. I'm not talking about listening to whatever weird bullshit they're spouting; I'm talking about listening to their pain and humanity.
The sad part is that many justice systems are designed to exclude those convicted of a crime, thus excluding the most vulnerable, who need inclusion the most. It's one reason why you have such high recidivism rates in societies with more vengeful justice systems.
Inequality is a form of exclusion, and we're wired to fight it. Fighting terrorism is fighting the symptom rather than the cause. The harder you hit them, the harder they fight back, because they've got nothing left to lose that they care about, and they have "righteousness" on their side.
Historically, when inequality goes on for too long, social unrest takes root and gradually grows worse, divisive factions and subgroups grow, "little battles" take place, and, if untreated for too long, revolution breaks out.
More equality does not mean communism or everybody gets exactly the same. You position is just a straw man argument.
> Scandinavian
Scandinavian countries are social democracies, like most of Europe, they are not community countries. And are no exception but good examples of implementing good policies.