North Korea has a very long history of breaking it's treaty commitments and violating diplomatic agreements.
That said, all-out massive attack isn't viable. there would be no way to disguise the political and military preparations for such an attack, and China wouldn't allow it anyway. Bear in mind the the South Korean capital Seoul is within medium artillery range of the border, and there are a lot of North Korean artillery pieces aimed at it. Even without a nuclear attack capability, North Korea could do them an awful lot of damage in ways that would be difficult to prevent.
NK falling is considered terrible by all parties. South Korea can't absorb it. China can't absorb it. It's a hot potato no one wants.