And Then There Were None red herrings ?? . . . . . .?

I read this novel years ago. If I had a copy in front of me I would have a fairly good idea of where to look. I remember people dying and then the shifty way the survivors would discuss the most recent death. I remember the older woman adopting a patronizing attitude towards the younger woman who was guilty of allowing the little boy to drown. So when the older woman died I remember thinking the younger woman injected the older woman and causing her death.