The term "bastard" is not an insult so severe now, but an ambiguity of language.
For those who prefer not to waste time on perfectionism scholars can consult the Wikipedia article devotes to the term "bastard" (1).
It will appreciate how varied social, legal and labor may occur at the discretion of whoever hears it sees or says it.
Most striking is that the expression allows both ends, as both can be offensive as complimentary.
We all know who uses that term offends, but also often used to indicate, ironically, the skill, alertness, readiness, the cunning of someone.
Now tell them an idea that relates to how this expression may be improving their position for cultural reasons.
The word slut is derogatory expression prostitute, ie woman selling sexual services.
Until recently (mid-20th century, approximately), the majority of households were supported by the parent, while the woman was by trade 'unpaid work', ie care of household chores, care children, elderly, sick, feeding, cleaning.
In this context, telling someone that her mother was selling sexual services could be radically offensive it was an honor for those women do not work for money, but all they did was out of love. Running any unpaid work in that context, was derogatory to the lady, her husband and other family members.
Since they had to leave to deliver the effort of her body in exchange for money, read: work, the situation changed.
In the present context, the term "bastard" not be considered a grave insult but a mere ambiguity in the speech.
Note: Original in Spanish (without translation by Google): El debilitamiento de un insulto.
(Este es el Artículo Nº 2.003)