3B

Why kill James Grant, and why was it important that Cotton died? Was James Grant naive in his dealing with the Knight?

Grant after revealing MI6 information from 1930 that could lead to the discovery of the Nostra Trinita he was no longer needed and was killed. His sharing the information showed he was naive in dealing with the Knight. Cotton having discovered the Knight's plot become a liability.

Grant was killed because he was no longer needed and after discovering the knight’s plot Cotton was no longer needed either.

Once it was determined that they had the information they came for, Grant and Cotton were expendable.

>4 Olivermagnus: agree. They were of no further use and the knowledge they had made them a liability

He had outlived his usefulness and letting him live would have been as >4 Olivermagnus:, said...a liability.

Grant was naïve and he paid the price. Whoever is leading this thing is cleaning up as he goes.

Yes, agreeing here. It brought me to mind of the villa owner - once he had revealed where the documents were, he had given up his sole bargaining chip.