"If someone I work with quits day-of, and there's no obvious signs that it was over an ethical no-go, I'm going to conclude that it's someone I don't want to collaborate with in the future."

Here's the problem, you really don't know the whole situation and making such a determination without data will be problematic. You could be punishing the innocent party. Worse, your former fellow employee might use the same lack of information to conclude that you agreed with the company over perhaps their very valid reasons.

As an example, your fellow employee's health is declining (not obviously to you but that is quite common too), work is keeping them awake (devops is so fun), and then maybe their mother got cancer. A quick meeting and no hope of relief is given, and they quit. Farfetched? No, sadly. They know they can go somewhere that will understand, and where you are isn't it. They're hundreds of these things, and not one will be told to you honestly by an employer.

Years later they get back to a happy place, and you do your "don't want to collaborate". We're in an industry with a lot of introverted people so don't expect them to give some explanation for your approval.

Disrespect should be truly earned and not inferred, or as my Dad taught me: come by your hatreds honestly and with full knowledge of the why.