If you hire someone who is currently employed and then fire them, you are doing very real damage to their finances and career. Empathy for employees is a valid reason to err on the side of avoiding bad hires over easy in/easy out.

I disagree. By optimizing to protect poor performers, you jeopardize all employees. It's more important that the company succeed and that people who are good at their job get to work unencumbered.

Framing the decision to fire someone as a trolley problem (cf. https://en.wikipedia.org/wiki/Trolley_problem) doesn't make you look any less cold-blooded :)

Have you ever been a manager? Again, it's not being cold blooded, it's one of the main responsibilities of the job. Being a leader isn't easy and you often have to make tough decisions and sacrifice in the name of the team. This has been well known throughout the history of leadership.