Biden didn't kill any soldiers. If you want to blame someone who's actions in the abstract increased the risk of attack. That would be Trump who freed 5000 Taliban soldiers, and then illegally drew the troops down below 5000, to 2500. That then created such a small force of US military in the country that they could no longer defend most of the country. That then forced either the US to reinvade - probably requiring an enormous surge in the 100,000 soldier range to restablize, or complete Trump's withdrawal. Since most of the US was in favor of withdrawal, Biden completed Trump's withdrawal.
The agreement with the Taliban is one of Trump's biggest (legitimate) mistakes. The agreement was poorly constructed, with absolutely no verification mechanisms or objective tests for the Taliban to meet and was being breached by the Taliban almost from the moment it was entered into.
The 5000 prisoner release is not a mistake, if you have an effective agreement that generates a cease fire and peace. To the extent that those 5000 are POWs they would be released anyway, to the extent they were war criminals they should not have been released. However, even war criminals are often part of the post-war amnesty to a certain extent - they may be subject to new trials afterwards.
Even after it was obvious that the Taliban were not living up to their commitments Trump continued to implement and support the agreement, even going so far as to criticise Biden delaying the exit from May 1st to Sept. 11.
But that said, excusing Biden for his role is complete nonsense. Biden's the President and he has been for eight months. The deal was failing before he took office, and Congress told Biden in February that the deal was failing and needed substantial reworking, including the introduction of objective compliance milestones and the ditching of the hard time table. Not even 3 weeks later Biden reconfirmed that he was going to continue with the deal and exit Afghanistan (intending on the May 1st line at the time).
At no point during his Presidency was the Taliban complying with the Agreement, and at any point he could have reversed the retreat based on breach of the Agreement. He choose not to do so, and he choose that deliberately. It's obvious from the leaked communications that he lied about his knowledge of what would occur. At no point was he incapable of ordering an effective response to maintain a safe evacuation.
The best spin I can put on it for both Biden and Trump is that they seem convinced there was no good exit, and that long term the Taliban will operate as a government and not just as a bunch of terrorists. They may even be correct, or it may just have been that they saw the writing on the wall that no matter when we left the Taliban was going to take over and there was no use in pissing them off if we weren't going to continue to occupy Afghanistan.
The triggering event was Trump drawing down the troops illegally and freeing 5000 Taliban which completely destablized the country.
The triggering events were many, starting with the deal Trump cut, including the election Trump lost (and the open acknowledgement of Biden's weakness), including Biden's repeated commitment to the "peace" plan and both Presidents ignoring the Taliban's violations. The 5000 release wasn't much of a trigger, but the release of the 400 that were held for crimes should not have occurred without real progress, or maybe at all.
The Afghan forces were capable of holding the country. But I think both Presidents came to realize that those forces would not do so. There was no commitment to the country, the soldiers weren't patriotic, they weren't committed to something because it was bigger than themselves, and they faced fanatics. Young men with passion for their homeland were more likely to join the Taliban to expel foreigners from their country than the defense force to get a piece of the foreign graft. Without a sense of a country or purpose no army can stand.
Let's put Afghanistan in a different context.
No one in the world had a better plan for the evacuation than the Biden Administration (if you disagree, show us the evidence).
Not clear what you "accept" as evidence, but how about -evacuate civilians before the military. Seems pretty straight forward. Mandatory evacuation of allies. Given what Biden knew, its pretty clear he knew the country would fall almost immediately - despite spreading a lie on that point - there's really no reason that we shouldn't have gone all in to make sure our Afghan allies got out. Unless of course you think "peace" with the Taliban is possible and worth it, in which case, you wouldn't want to issue any new aggravation during the exit even if the Taliban acts moderately terrible. That's Real Politik at its worst, but that may be what occurred and why Biden is willing to lie about what he could have done and declare this a success.
Among national political figures, no one had better insights a decade ago that the Afghan regime was never going to be able to stand on its own than Joe Biden, who was the loudest voice in the Obama Administration for reducing our involvement then.
Assuming you believe that, and there are other less flattering reasons to believe Joe had the position he did, doesn't that just further compound Biden's guilt for lying about the collapse? If he had no illusions, then his pushing the Afghan President to put out false information about how well things were going seems callously designed as an attempt to keep a lid on things just long enough for him to pull out and leave our allies to face the firing squads. It would literally look like he sold out all of our Afghan allies to try and save just the citizens and soldiers based on the callous knowledge that Americans care about dead Americans far more than even thousands of our non-American allies betrayed in murdered.
That's not moral courage.