As usual with this discussion, the problem is that the question "is this evil?" needs more context. Are kobolds intrinsically evil? Almost always evil? Is "evil" an observable status? Is there an objective definition of "evil" in the world? Are all creatures equal in the sense of having souls? Life after death? Self-awareness? Free will?
Most campaigns exist in a world where creatures that talk are essentially humans in rubber masks, so we expect that our-world ethics apply to them. In our-world, if we have a terminally ill bunch of children who pose a danger to the community, we isolate them and provide palliative care to ease their pain, but we do not kill them.
A good cleric working under our-world ethics would stop adventuring and take care of them, allowing them as good a quality a life as possible (the fantasy book Name of the Wind has a cleric who does exactly this in an urban setting).
But this is where our-world ethics conflict with a fun game. It's hard to justify the typical adventuring lifestyle for any character with our-world good ethics. So we bend the ethics. One way is the old-school way of defining some creatures as intrinsically evil, so we can treat them as we would viruses and kill them on sight at every stage of development. Another way is to fudge the definition of good to be "our-world good, but limited by what makes the game fun to play".
Most modern fantasy campaigns lean to the latter, so your cleric is performing a good action. He is doing that action that most conforms to our-world ethics (alleviate suffering), while constrained by the game needs of not abandoning the party.