Did Suwako do anything prior to her "death" to make Cirno want her dead? Or did Cirno go overboard on something?
Was Suwako in poor/less then prefered physical condition when Cirno killed her? Hungry, thirsty, tired, or is this all Irrelevant?
Did Suwako notice that Cirno was coming to kill her before it was to late? Or was she unaware of Cirno until she already attacked.