Good people doing bad things

I came to write The House at Riverton,
first and foremost, because I am fascinated by the eternal question of

what makes ordinary people
do terrible things?.

What might drive a woman to kill her lover?

To betray a beloved sister?

And what happens to tragedy's survivors?

To those unknown witnesses whose guilty secrets whisper in the recesses of their mind, refusing to remain forever repressed?

What might induce such a witness to divulge her secrets?

And to whom might she make such a confession?