So either Usogui just killed a person because of a grudge, or 13 is not correct.
But based on Suteguma's reaction, Usogui should know he has 6 or 7 beads.
And I can't think of a single scenario where Usogui would press 13 while knowing this.
Only one which makes a little bit of sense is if Usogui has 10 beads and pressed 13 to lead away from it.