Do you think buying and using souls in larval form for spell components is evil?
I'd say yes, it's evil. Although I can also imagine rather amoral or pragmatic neutral (N)PCs doing it on occasion (it might help them to justify their deed that the larvae are by definition evil souls).
There is a bit of a theological question in the background here: namely, is what fiends do to damned petitioners an evil act if the righteous gods have decreed that the petitioners are to be sentenced to damnation?
The answer, I would say, is yes. Having evil be done to them is part of their punishment. Hence the PCs should be reluctant to do anything that they see fiends doing.
The opposite answer, that any cruelty perpetrated on the damned is justified, leads to any number of morally dubious situations, IMO, such as mistreating damned petitioners for no good reason.
Finally, a middling answer might be that what fiends do is no guide to what mortals should do, and treat the buying and selling of souls as one would the mortal slave trade, and their sacrifice in the manner one's society treats human sacrifice. If one is from a Greek or other Bronze Age background slavery, such as binding such a soul to labor, might be permitted -- but sacrifice is usually not, unless one is, say, Mayan.
Binding a damned soul to service might be viewed differently by different pantheons. Some might regard it as a kindness -- hey, you're a better taskmaster than a fiend! Others might regard it as interfering with the proper damnation of an evil soul, like a jailbreak.
I agree that various pantheons and cultures would have a different view, then maybe what would guardinals or similar creatures think? Most of the punishment is undeserved and does not accomplish anything in service to good. This is killing fiends before they could develop into something dangerous.
The guardinals seem rather passive and defensive as concerns the battle of good versus evil to me. I would say that, just as when dealing with any other being in the universe, they would condone killing only in cases of life or death. Pre-emptively destroying a damned soul so that it does not become a fiend does not strike me as a morally upright thing to do. The soul is sentenced to damnation, not execution. It is especially not right to use the soul for magical power and then claim that your act was justified by preventative measures against something the larva would have done in the vague and distant future.
Now, I would agree that the fiends also ought not to be torturing the souls (probably -- see theological arguments). But... as long as evil is being done to living mortals with souls the fate of which are still in doubt, there are usually far more important priorities on which to spend one's time and energies than attempting to protect the damned from fiends.
Although you now have me picturing very small outposts of celestials on Acheron and Pandemonium, where they tend to souls that are only slightly evil and, they perhaps believe, have yet a chance of being miraculously redeemed into true LN or CN states.
Come to think of that, if that were even a remote possibility, you might get such operations all over the planes. Maybe Gehenna has recruiters for Baator, offering souls the chance to sign a divine contract with the devils that will allow Baator to claim them, or places on Gehenna that shelter runaways from Baator. Planes probably don't like giving up souls without a fight, so both of these moves come with costs so high as to, in the end, make them not really a good deal for the souls, of course.
It's possible there are outposts of ''redeemed'' fiends trying to lure celestials, similar to the Garden in Baator.
Using souls for "stuff" seems like a perversion of nature thing to me.
At least, taking other souls and using them (rather than, say, good people willing using their souls to energize something).
Similar to creating undead (though that depends on how you fluff it too... I have mixed feelings about the "undead = evil, period, and good dead things are something else" idea that D&D materials started to adopt.
I'd say in the planar set-up, the evil souls aren't really in the lower planes to be "punished" ... it's more your alignment naturally draws your soul to the properly aligned plane, so it's a natural consequence.
(Though obviously it will vary...)