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It is quite possible to study insects without collecting them, but you have to choose insects that do not need microscopic examination to identify them. Most butterflies and macro-moths, grasshoppers and crickets can be field identified with practice. Even some more critical groups can be studied purely in the field, as long as you accept that there will be many species which you will not be able to identify. But if you fancied studying hoverflies, you could choose to study Eristalis, Volucella, Criorhina for example, most of which can be identified in the field by capture, inspection and release.

In general though I would not be concerned about killing insects. The adult phase of most insects is very short lived in any case. It is really just a stage, whose only purpose is to mate and reproduce. It has lived 90%+ of its life, by the time you capture it. Often, a quick death caused by an entomologist may be far more merciful than deaths that nature deals out - dissolved and sucked dry by spiders, paralysed by wasps and slowly eaten alive, devoured from within by a parasite etc.

The contribution to knowledge, that taking a specimen makes is immense. The conservation of many species is absolutely reliant on our knowing what they are and where they live. In most cases some specimens have to be taken to be sure of such details. So, yes, collecting is absolutely an acceptable practice. Only the mass collecting of specimens for commercial purposes, or to display pretty series of variations should be condemned, as this contributes very little to science and conservation.

Personally, like the vast majority of entomologists, if I am confident of an insects ID in the field, I will always release it, if it is not essential to have a voucher specimen.