The central moral evil in interrogating someone by means of torture is that it overrides the victim’s moral agency. That is, the whole point of the exercise is to render the victim incapable of moral self-governance, so that your will, the will of the torturer, becomes entirely sovereign. This is intrinsically wrong both in Aristotelian/Thomist thought and the various moral philosophies derived from Kant. Since, in classical moral philosophy (and indeed in the moral reasoning we use in everyday life) you can’t perform an action that is intrinsically evil in order than good may come about, you can’t torture someone to get information out of him even if that information would save many lives.
There are more factors at play, including what kinds of violence are morally permissible in wartime, but respecting everyone’s moral agency is the most basic moral principle at stake.
As a side note, you would think that this would all be clear to the good people at First Things, who’ve read their Aquinas and their dogmatic constitutions. I know First Things is against torture. I hope we hear some clarification from them soon.