-person is suffering mental health issues due to strain of job.
-persons performance drops rapidly without warning as a result.
-boss takes workload off them without putting them on a pip process, to give them time to work through their issues.
-person quits right after stocks vest which upsets boss who put extra effort into helping them
-person gives interview to justify their behaviour
If a job gives you mental strain, especially if it is affecting your performance, IMO the right thing to do is to quit. I'm not surprised the boss was disappointed, but I do think they should've seen it coming.
I've been in situations where a job evidently isn't for me anymore, albeit in a seemingly healthier environment— my boss and I talked openly about the team no longer being a fit for me for weeks if not months prior.
I was evidently not working at my full potential too, and when I told my boss I was quitting, they told me what date I should officially submit my resignation so that I'd still get my latest bonus. It was all very amicable and objective.
That's one reading, but unless we assume that OP is straight up lying in their description of events, the boss was lying to them about whether they were on a PIP or not, and had given them no indication that their performance was flagging until suddenly doing this to them.
It doesn't sound to me like the boss put in extra effort.
People often forget that corporations are just a group of individuals at their core. If every one is as shitty as a human being as the person giving the interview, no wonder Amazon is so horrible.