There's a difference between disappointment about a product (which is fine) and doing an experiment to test that might knowingly create bad feelings in someone's life in one cohort of your experiment.

What if an almost-suicidal person got the "b" version?

A/B testing should be used when you think, in good faith, that BOTH "a" and "b" versions could be good and want to know which one works better, not to confirm a hypothesis that "b" has a negative impact on users' personal lives.