Why did Jin Yong write Fu Kangan so viciously?

The real Fu Kang 'an is fierce, a soldier and brave. It is not as vicious as described in the novel, and during the Qianlong period, Fu Kang 'an was highly valued by Qianlong. And it was also written in some historical materials later. Fu Kang 'an was born in a noble family, with a good family background, and he was even more talented. Some people commented that he was an important general in history because he made great contributions to the stability of the frontier.

Fu Kang 'an is the son of Fu Cha Fu Heng, who was Fu Heng in the previously popular Yan Xi Palace. Therefore, Fu Kang 'an's background is quite noble. More importantly, Gan Long likes Fu Kang 'an very much and regards him as his own son. So later there were rumors that Fu Kang 'an was the illegitimate child of Gan Long, but personally, it was just a rumor, not true. He and Gan Long were like father and son.

Moreover, for the cultivation of Fu Kang 'an, Gan Long has made a lot of efforts, and he is more careful than cultivating his own son. Maybe this Fu Kangan is really pleasing to Qianlong. Of course, it may also be because Fu Kangan himself is smarter and more powerful, so he is so favored.

Many plots in the novel can't be taken seriously, and so can many characters. It's just made up by the author for the story. You can only read novels, not historical facts. Because the author is subjective and has his own likes and dislikes about this character, he writes this character according to his own ideas. For example, Fu Kang 'an, I guess Mr. Jin Yong didn't like it very much, so he wrote a vicious person.