I think it’s really sad when a business has to experience a bitter person or worse a bitter customer. There’s nothing the business can do to make this person happy that person is just mean and miserable for no fault of the business.
If that business has un-informed or reactive management then the employee on the receiving end of that unhappy person might get counseled about how to be a better employee or how to improve their people skills when probably the exact opposite is true.
I think the same is true with online and customer reviews. If you happen to be a business just starting out and you happen upon these kinds of “customers” at the beginning of your business you might in trouble because you’re never going to sell to the silent majority of people that might actually enjoy what product or service you’re offering.
Not really sure how to handle these kinds of people in general, just hope you are doing things to attract people on the other end of the curve to cancel them out.