sin, crime
辜 centers on moral wrongdoing: a sin or a crime, carrying a sense of guilt and transgression.
辜 combines 古 (old) and 辛 (bitter, punishment), suggesting an old or established offense. The exact historical development is uncertain, but the components point to wrongdoing or penalty.
An old (古) bitter punishment (辛) for a sin or crime: 辜 is the guilt that brings harsh consequences.
For コ, imagine a criminal committing a sin and then calling out 'co!' in regret: co -> コ, the sound of guilt.