Massacre Wurm enters, it's first ability triggers, giving all opposing creatures -2/-2 until end of turn.
All of your opponents' lands die as a state-based action due to having zero or less toughness, as they are creatures due to
Life.
When the lands die,
Massacre Wurm's second ability triggers for each land that dies this way, causing each opponent to lose 2 life for each land that died under their control.