Those of us seeking an answer for Krishna’s act (of not saving Karna) are appealing to his mortal side not the divinity he truly is. One of the key teachings from Gita —
karmaṇy-evādhikāras te mā phaleṣhu kadāchanamā karma-phala-hetur bhūr mā te saṅgo ’stvakarmaṇi
While many explanations exist for this one of the most famous teaching of Gita the essence is.
Do your duty and not worry (get distracted by) about the outcome, because it’s not in your control.
Yet he says another interesting thing in 3.22 and 3.23 – I need not do anything yet I do my work, for if I don’t, people shall follow me (doing nothing) . If I ceased to perform prescribed actions, all these worlds would perish. I would be responsible for the pandemonium that would prevail, and would thereby destroy the peace of the human race.
Is this caring for outcome?
I think this is the clarification. Not worrying about outcome is not distracted or attached to the outcome.
When you are not attached to the outcome its not selfish, but greater good.
And thus to save someone or not was a question of greater good – greater than saving a few lives.
So the question is why Krishna should have saved Karna?
A detailed related discussion on this video of mine here:
(This post is duplicated from my earlier answer posted via a second account. I never realized I had a different login.)