It took Diana two minutes to calm herself down and composed herself after the emotional breakdown that possessed her to something that she wouldn't normally do.
Alexander found her action to be understandable. If the person who killed his parents is sitting across from him, he too would lose his rationality and begin attacking them in a way unimaginable.
Anne was an executive member of the Black Hand, which would make her an accomplice to the death of Alexander's parents. If not for his state of existence that he is not the real Alexander, there's no doubt, instead of stopping the fight, the real Alexander would be even glad to join Diana.
