Why was Armstead killed?


Hint 1 of 6:
Did you notice that Armstead's sword is normally kept out of reach?

Hint 2 of 6:
Arnaud waited for him to get the sword; the average murderer doesn't wait
for his victim.

Hint 3 of 6:
That wait suggests that Arnaud wanted to duel Armstead.

Hint 4 of 6:
It was probably a matter of honor.

Hint 5 of 6:
Do you remember the letter in Arnaud's hotel room?

Hint 6 of 6:
It was written by Arnaud's sister, as the man in the theatre suggested.