Who killed General Farnsworth Armstead?


Hint 1 of 7:
Start by ruling out suspects who have an alibi or don't have a motive.

Hint 2 of 7:
No one involved with the Polar Star Diamond can be guilty; Armstead was
dead when Rostov arrived, and Carson Cabot was at home all morning.

Hint 3 of 7:
You can also rule out Lord Fitch; his wife was sick, and he stayed with her.

Hint 4 of 7:
Booth Lacey was at St. Mary's, Wentworth Cobbett was writing, and none of
the Tontine survivors had a strong motive to kill Armstead.

Hint 5 of 7:
Only one person had a reason to hate Armstead.

Hint 6 of 7:
A recent event triggered the murder.

Hint 7 of 7:
A death in his family, as a matter of fact.