"Such an additional crime was not a part of the original plan.

There was no apparent necessity.

Your estate was about to be settled finally, and given over to your control in accordance with the terms of your father's will.

Hobart must have known all this from Percival Coolidge, and exactly what steps must be taken to secure it.

Once the money, and other property, were delivered to the fake Natalie, the cashing in and get away would be easy; even the identity of the thieves would be concealed.
Killing you was not at all necessary to the success of their scheme." "But they did try to kill me." "Yes, later, by the sinking of the yacht.