How many times have you seen the bumper sticker "If you want peace, work for justice"? And what do you think the person who pasted that sticker had in mind by "justice," anyway? If you use the other stickers that share the bumper to triangulate, "justice" seem to mean either (a) the abolition of extreme poverty, (b) the abolition of extreme ethnic prejudice, or both.
I'll start with Krueger. What Makes a Terrorist? takes a standard social science look at the hypothesis that poverty causes terrorism. He has a very one-handed answer: No. Individual terrorists are, on average, well-to-do for their societies. They are neither desperate nor lacking in years of education.
If terrorist organizations are small, they tend to be composed of the elites who care deeply about the cause. They are the first ones to join. For the movement to grow past a certain point, it must recruit a wider pool of members because the truly committed are already involved. Beyond a certain size, the additional recruits tend to be motivated more by pay and less by ideology; these tend to be people of lower socioeconomic status.
Furthermore, Krueger finds little evidence to support what he calls the macro-level "Robin Hood" theory that terrorists are "motivated by inequality in their societies or by the poverty of their countrymen." Krueger says that the data suggest that it is lack of civil liberties, not lack of income, that predicts terrorist activity (though in the Q&A he admits that reverse causation may be at work).
Krueger is a little quick to dismiss some vague but plausible popular claims about terrorism. Case in point: "[A]ssuming those who attack us do so because they are desperate or because they hate our way of life provides a reassuringly simple answer to a disturbingly complex question." But hold on a second. "Desperate" doesn't just mean "hungry," and there's a lot more to "our way of life" than our income. I daresay that terrorists hate something important about our way of life, and that you have to be desperate in some important way (e.g. "desperate to win") to sacrifice your life for a cause.
One last point: Unless I've missed something, Krueger pays little attention to a very different "reassuringly simple answer" to the problem of terrorism - namely, that terrorists hate us because they don't like our interventionist foreign policy. (He does include "Occupier" and "Occupied" dummy variables in some regressions, but these just scratch the surface of interventionism). And frankly, I suspect that if we checked the correlation between "satisfaction with U.S. foreign policy" and either support for terrorism or terrorist activity, it would be highly significant and negative for both individuals and countries.
It wouldn't surprise me if satisfaction with U.S. foreign policy decisively beat Krueger's measures of civil liberties in a terrorism prediction horserace. But even if our stories had to share the glory, it wouldn't matter that much in terms of policy advice. After all, there is a lot that the U.S. can do to make other countries more satisfied (or at least less dissatisfied) with its foreign policy. And there is very little the U.S. can do to bring civil liberty to Saudi Arabia.