The reason “bastard” became a derogatory term, rather than merely a descriptive one, is that the parents were too immoral/undisciplined to marry before reproducing, therefore were assumed (generally correctly) to have been unable/unwilling to teach morality/discipline to their child.
No, the reason the term became derogatory is because people search for the most disrespectful term they can use to throw out at people. They insult their mothers, their status without regard to the power of words in an effort to hurt.
They never stop to think that a baby is absolutely innocent with no control whatsoever of the events surrounding its birth.
It would be better to insult the parents and leave the result of the actions of guilty parents free to live a life with honor or dishonor cast on the child based on the child’s actions.
But - again - the only reason for the word is to hurt and who is to expect they should want to be fair in their quest to hurt?