It refers explicitly to the burden that Caroline has carried for so many years, ever since, in a fit of uncontrolled adolescent rage, she hurled a paperweight at her baby sister and injured that sister for life.
Now, at last, she has the opportunity to pay the debt she owes.
And if it is any consolation, I will say to you all that I earnestly believe that in the payment of that debt Caroline Crale did achieve a peace and serenity greater than any she had ever known.
Because of her belief that she was paying that debt, the ordeal of trial and condemnation could not touch her.
It is a strange thing to say of a condemned murderess - but she had everything to make her happy.
Yes, more than you imagine, as I will show you presently.
"See how, by this explanation, everything falls into its place where Caroline's own reactions are concerned.
Look at the series of events from her point of view.
To begin with, on the preceding evening, an event occurs which reminds her forcibly of her own undisciplined girlhood.
Angela throws a paperweight at Amyas Crale. That, remember, is what she herself did many years ago.
Angela shouts out that she wishes Amyas was dead.
"Then, on the next morning, Caroline comes into the little conservatory and finds Angela tampering with the beer.
Remember Miss Williams's words:
'Angela was there.
She looked guilty.' Guilty of playing truant was what Miss Williams meant; but to Caroline, Angela's guilty face, as she was caught unawares, would have a different meaning.
Remember that on at least one occasion before Angela had put things in Amyas's drink. It was an idea which might readily occur to her.
"Caroline takes the bottle that Angela gives her and goes down with it to the Battery. And there she pours it out and gives it to Amyas, and he makes a face as he tosses it off and utters those significant words -
'Everything tastes foul today.'
"Caroline has no suspicions then, but after lunch she goes down to the Battery and finds her husband dead - and she has no doubts at all but that he has been poisoned.
She has not done it.
Who, then, has?
And the whole thing comes over her with a rush: Angela's threats, Angela's face stooping over the beer and caught unawares - guilty - guilty - guilty.
Why has the child done it?
As a revenge on Amyas, perhaps not meaning to kill, just to make him ill or sick?
Or has she done it for her, Caroline's sake?
Has she realized and resented Amyas's desertion of her sister?
"Caroline remembers - oh, so well - her own undisciplined violent emotions at Angela's age.
And only one thought springs to her mind: How can she protect Angela?
Angela handled that bottle - Angela's fingerprints will be on it.
She quickly wipes it and polishes it.
If only everybody can be got to believe it is suicide. If Amyas's fingerprints are the only ones found.
She tries to fit his dead fingers round the bottle - working desperately, listening for someone to come. "Once take that assumption as true and everything from then on fits in.
Her anxiety about Angela all along, her insistence on getting her away, keeping her out of touch with what was going on.
Her fear of Angela's being questioned unduly by the police.
Finally her overwhelming anxiety to get Angela out of England before the trial comes on.
Because she is always terrified that Angela might break down and confess."
Slowly, Angela Warren swung around.
Her eyes, hard and contemptuous, ranged over the faces turned toward her.
She said,
"You blind fools - all of you.
Don't you know that if I had done it I would have confessed?
I'd never have let Caroline suffer for what I'd done.
Never!"
"But you did tamper with the beer," Poirot said.
"I? Tamper with the beer?"
Poirot turned to Meredith Blake.
"Listen, monsieur.
In your account here of what happened you describe having heard sounds in this room, which is below your bedroom, on the morning of the crime."
Blake nodded.
"But it was only a cat."