I think millions of years of natural selection are a better advisor than a stranger on the internet, or teachings of the status quo.
We have the innate capacity to love and hate, to feel attracted or repulsed etc.
If the capacity to be bitter has survived millions of years of natural selection, perhaps it serves a function, however hard we tell each other to look down on it.
Evolution has also produced fatal cancer in infants, among other miserable horrors. The existence of a property in evolved creatures does not imply that it is adaptive.
Clearly everyone possesses the capacity for bitterness, as they are able to agree on its meaning. Otherwise they couldn't possibly know what the word means.
Clearly natural selection has selected for this capacity.
You pretend to disprove the adaptive nature of a trait by trotting forth "fatal cancer in infants" which clearly natural selection would not systematically select for without good reason. Perhaps artificial human doctrines have the capacity to selectively advantage "fatal cancer in infants" but brutal natural selection wouldn't. Natural selection is the abacus of the grim reaper.
You may call it a dark calculation, but do you know whats worse than a "fatal infant cancer"?
Two or more cases of this "fatal infant cancer"!
I know almost no one who has "fatal infant cancer", yet I have never met a person without the capacity for bitterness.
We have the innate capacity to love and hate, to feel attracted or repulsed etc.
If the capacity to be bitter has survived millions of years of natural selection, perhaps it serves a function, however hard we tell each other to look down on it.