It helps to first clarify exactly where kammic effects could manifest.
We are talking about committing murder.
For the kamma of taking a life to be “complete”, the following four steps need to be completed: there must be a living being, one must know that it is alive and one must have the intention to kill that being, one plans and carries out the necessary actions to kill, and finally, the living being ends up dead.
-If all necessary steps are completed, then it is called a kammā patha. That will bring the full kammic force of taking a life.
-The more parts completed, the greater the effect.
C would not be involved in the activities of Group A if they took place.
– So, C would not be responsible for any of the four steps, except for the last step. It is possible that C could have prevented those murders. Of course, since C would not be involved in the killings in any way, the effect could be less for that step.
The more serious issue could be the issue of compassion. C could prevent a large loss of life by exposing the plot.
– If C did not warn, and if a lot of people got killed, that could haunt C for a long time.
It is not easy to resolve the kammic consequences of complex cases like this.
Unless this is a real issue, it is not even worth to contemplate too much on “theoretical” cases like this one.