According to his wife, there was no one to leave him a legacy.

She never heard of his receipt of this money from any source.

But--there's the fact! What explains it?
My theory--that the rumour that Collishaw, with a pint too much ale in him, had hinted that he could say something about Braden's death if he chose, had reached Braden's assailant; that he had made it his business to see Collishaw and had paid him that fifty pounds as hush-money--and, later, had decided to rid himself of Collishaw altogether, as he undoubtedly did, by poison." Once more Bryce paused--and once more the two listeners showed their attention by complete silence.
"Now we come to the question--how was Collishaw poisoned ?" continued Bryce.

"For poisoned he was, without doubt.

Here we go back to theory and supposition once more.