Guilty people need to end the stress they suffer from lying, this is why we get deathbed confessions, to ease the soul of the guilty person.
A guilty person can never ease the stress they suffer, this is why, sooner or later, they confess to someone.
It eases their guilt and relieves their internal stress.
Also, a secret once shared, even if with one other person is no longer a secret.
The mccanns have the fear of not only leaking the truth themselves by
accident, they also have the fear that one or more of the tapas 7 will
admit what happened to ease their own guilty conscience.
None of them were close friends, at most, they were acquaintances with the mccanns.
Why would they keep such an horrific secret for 9 years?
If they had no involvement in the actual deed why stay silent?
If they were doing other stuff be it bonking each other, illegal drugs
etc that would be nothing, who cares who they were bonking?
Who cares what drugs they took (Drs are well known for 'self medicating')
What if they were getting drunk every night, who cares.
None of that would be important, it might mean a few snickers from friends and neighbors and maybe a scandalised parent.
For them to stay silent for so long has to mean something more serious was taking place.
Something so serious they had to keep it silent.
What could be so serious as to silence 9 people?