However if you read what I wrote:
1) Current attitude is not necessarily prior attitude
2) A histrionic tirade is not indicative of an outwardly perceptible attitude, in fact, its more common that these kinds of outburts are from a person who is not outwardly bitter enough day-to-day and is forced to be positive. (thus it boils inside them and becomes venomous)
3) Bitterness is usually the combination of a (often still) motivated person who feels let down. Your companies most negative voices are very often the ones who are passionate but sad about things. Its the “checked-outs” who you really don’t want if you’re building something you want to be good.
You're applying the circular reasoning of "of course I treat you like crap because you have a bad attitude (because I treat you like crap)", while ignoring the part in the bracket.
This individual was a great contributor and long outlasted my tenure at the company (so it's not just my bias), only to eventually move on to even better roles.
Frankly, if you work in tech and haven't been bitter about some nonsense in this field, I suspect you must not be particularly engaged or passionate about the area you work in.
It doesn’t necessarily speak to the parent’s attitude before becoming bitter. Hard to draw such a direct conclusion for me.