maybe black karp represent evilness?

Born to backstab anyone who helps

indeed

No, that is not 100% accurate.
Born to backstab those who helps them repeatedly to death, then devour them and doesn't even leave bones (no evidence left)

Carp can become dragon. Black Dragon is easily evil and treacherous. Black Carp is someone who has potential to become insanely powerful but will surely betray and destroy all who helped it in order to advance even further, leave no evidence, start anew to join someone even stronger so that it can do the same.

Last episode and yeah what a story 😑