The implied setting of the default D&D game tells us that the souls of dead people go to their respective afterlives dependant on their alignment. The various outsiders are implied to spring from the ranks of these souls and almost always have unchanging alignment. Okay, I'll say it, imo, the living life determine the immortal afterlife - so to live is extremely important but the afterlife is the main course.
Killing a Deva to save present lives and future lives yet unborn, can be approached in many ways.
First off, I'll say that killing an innocent (alignment aside) is an evil act that cannot be dismissed. Now I may concede that it icould be necessary and the most correct thing to do but the act in itself is evil, full stop.
Secondly we come to the value of a life * quantity. If this Deva's life was worth $1,000,000
and so was every individual on the planet that was facing oblivion, then the numbers direct to the correct course of action and the Deva would even volunteer it's own destruction given the strong embodiment of goodness that it is. However, if we say a life is priceless then we have the problem of infinite*1 = infinite*6,000,000,000 (e.g.).
Thirdly, the differentiation between mortal lives and immortal lives, if this is going to apply to the campaign (and is dependent on the afterlife set-up). If the loss of a planet merely allows all the souls to enter the afterlife while the loss of a Deva is total oblivion, we could think that it is better to save the Deva. The tragic loss of a whole mortal world could be a drop in the ocean compared to the unending grief felt by all good souls at the loss of their brethren.
Fourthly, the lives of future generations may or might not enter into the equation. Certainly there is the potential for them to eventually live their lives but they do not currently exist (unless the campaign has a 'unrefined soul source', which I had in a previous cosmology).
Is killing a good or bad action? In the best of circumstances it may be highly regretable and solely anguished over afterwards, but necessary for self preservation. The protection of an innocent is a good thing, and the self is not excluded in this regard, the fact that protector and innocent are one does not invalidate the goodness of the action.
As mentioned prior, I would examine the action, intent, and circumstances on a case by case basis before deciding.