These are troubling things.
The main part of the moral responsibility belongs to the Afghan leaders who took the bribes.
The fact that their army didn't fight is also a thing.
Should we feel guilty, instead of merely sad, if people die?
I am not sure. Does the responsibility return to us because we might have done something when the Afghans failed to? Or have we discharged our responsibility because we gave them a chance to act responsibly?
I feel most guilty for the Afghan children who grew up thinking there was hope for a better future and whom we have abandoned. Might that generation have ended the awfulness of Afghanistan? Given the chance, I think they might have. But they needed another 20 years to take over. And no one wanted to pay the price to secure their future.
For me, the least bad option was to stay. I think we will have their blood on our hands. And that it would have been worth the price of staying.
But I am not so certain of the least bad option in this case. But my judgement is that staying in the small numbers we had committed was better than leaving.