Together they found the strychnine they were looking for--not very much, but the proportion that was combined by Shillito with less traceable drugs to make the death process more rapid--and quite overlooked the signs of cancer in the liver.
The outcome was that Lady Shillito at the inquest found herself "in a very unpleasant position" and was placed under arrest, and later charged with the murder of her husband.
Believing herself guilty she summoned all her resolution to her aid, admitted nothing, appealed to Michael Rossiter and others for advice.

Thus David was drawn into the business.
[But this doesn't sound very credible, you will say.

"If the husband felt he could not face the agony of death by cancer, why didn't he leave a note saying so, and every one would have understood and been quite 'nice' about it ?" I really can't say.
Perhaps he wished to leave trouble for her behind him; perhaps he divined the reason why she thought a day nurse unnecessary, and insisted on giving him his day medicines with her own fair hands.
Perhaps he hoped for an open verdict.

Perhaps he wasn't quite right in his mind.

I have told you the story as I remember it and my memory is not perfect.