But Bakkot's claim was that our culture should allow it, which should not be expected to increase the number of psychopaths.

First, any single relaxed taboo feels to me like a blow against the entire net of ethical inhibitions, both in a neurotypical person and in a culture (proportional to the taboo's strength and gravity, that is). Therefore, I think it could be a slippery slope into antisociality for some people who previously behaved acceptably. Second, we could be taking one filter of existing psychopaths from ourselves while giving the psychopaths a safe opportunity to let their disguise down. Easier for them to evade us, harder for us to hunt them down.

I'm also not sure that unbounded social stigma is an effective way to deter people who essentially don't care about other people.

Successful psychopaths do understand that society's opinion of them can affect their well-being, this is why they bother to conceal their abnormality in the first place.