Elise never intended to protect somebody at the masquerade party, rather she used them to lure Katarina
As pictured above, LeBlanc coordinated plans to Elise for her to capture Katarina, a skilled assassin. We don't know the reason why the Black Rose has an interest in Katarina, but I do believe that Elise laid a trap. Elise never intended to protect the person Katarina assassinated, but rather used them as bait and trapped Katarina in her web