Look at the context for the track record: was it against all odds, or were they raised with a silver spoon? It is much easier to get ahead when having good loving parents, good teachers, good friends, but there might not be much of a fighter in those champions.
and definitely: people who bad-mouth others will bad-mouth you when you leave, no matter how noble you behave. But you still have to be noble to avoid your actions coming back and biting your ass.
how about "people will bad-mouth you." full stop. There is nothing you can do to stop it.
Now, how you respond says a lot, especially if you own a business, and people are bad-mouthing you on 'the permanent record'
1. don't respond as if you are taking it personally.
Sure, being attacked unfairly by a customer feels like a fist to the gut. But it's not. This won't burst your spleen. Man up and act rationally.
2. acknowledge problems the badmouther brings up, if some of them are valid complaints. Explain how you are trying to prevent those problems from happening again. If possible, offer a refund, etc.
3. try to deflect invalid complaints without getting angry.
Fear is not the mind killer. Anger is the mind killer. If the badmouther is being unreasonable, it's likely that others will see this as well, (If others don't see this... perhaps your idea of reasonable needs to be re-calibrated?) If you can't deal with this in a calm manner, don't say anything at all. personal attacks in response to personal attacks will make you look like a 15 year old or a politician, in any case, nobody anyone wants to do business with.
and definitely: people who bad-mouth others will bad-mouth you when you leave, no matter how noble you behave. But you still have to be noble to avoid your actions coming back and biting your ass.