Why are certain words bad where as others are not and who got to decide the other name for a donkey was bad or a female dog or even the F-word were considered bad words?

Answer 1

The original bad words, curse words, were socially 'frowned-upon' because of the danger they were believed to instill. As people would often use these words as an expression of offense, other words that indicated similar emotions were also considered 'bad words'. As time progressed, the degree of use of the words dictated whether or not it remained offensive, and the bad words became taboo, both through the tradition of curse words, or swear words, and more simply through the offense derived through their use.
A modern-day example is the word "spastic", which was considered a proper term for someone of physical disability 30 years ago, and is now a 'bad word' owing to its repeated use as an insult to members of the physically disabled community.
Additionally, the term "bitch", in reference to a female dog, was a bad word, not necessarily derived from use and consequences in context, but instead resulting from a combination of a reference to a man's mother and her resemblence to a dog, using the term metaphorically to mean "she looks as would a female dog". The term has adjusted in its offensive meaning to mean a female of behaviour which offends the user of the term, when in an informal and colloquial context, through common use as an insult and progressive misinterpretation.