Treachery, an aggravating circumstance

Dear PAO,

My sister's friend was a victim of a stabbing incident. The assailant pretended to be a customer of her food kiosk, and, out of nowhere, he stabbed her two times with a knife. My sister was told the assailant was already carrying the knife when he went to the kiosk. The victim's family wants to pursue a case against the assailant, who, as it turned out, is the brother of the owner of another food kiosk in the same food compound. There seems to be an unfriendly competition between a few merchants there. The family wants to know if the swift and unexpected attack can be considered a form of treachery.