Striving to explain his reasons at that distance of time, he said it was Robert Turold's last remark which really decided him--did the trick, as he phrased it.

Actually it must have been a prompt recognition of the kinship between two lawless souls.
He left his seat and went across to where the two young Englishmen were earnestly talking, unaware that they had been overheard.

He approached them as one shipwrecked sailor might approach two other castaways marooned on the same rock.

They all wanted money, and they all wanted to get away from that God-forsaken hole.

Diamonds they were after?
Well, he could take them to a place at the other end of the world where there were enough diamonds in the rough to make them all rich for life.
After the first surprise at his interruption they heard him in silence, and then plied him with questions.