Nothing should swerve him from making these plans as perfect as it was humanly possible.

He would be on hand to search Karlov himself.

If the drums were not on him he would return and pick the old mansion apart, lath by lath.

Gay old ruffian, wasn't he?
Another point worth considering: He would keep his discoveries under cover until the hour to strike came.

Some over-zealous subordinate might attempt a coup on his own and spoil everything.
He picked his way to the far end of the cellar, to the doors.