朝鲜停战协定 is just 停战协定, not a peace treaty. One side can unilaterally nullify 停战协定 without violating international law. The Hague II (1899) Treaty, says "If [the armistice's] duration is not fixed," the parties can resume fighting (Article 36) as they chose, but with proper notifications. Thus North Korea simply notified ROK that they nullifies 朝鲜停战协定, and wasn't condemned as violating international law.
http://en.wikipedia.org/wiki/Armistice
Enduring peace on the Korean peninsula would require both sides to sign a peace treaty. With China still backing North Korea, this is unlikely to happen soon.