Scripts in WME are always owned by some game object. So when the object is instantiated, its scripts are started, and when the object is deleted, its scripts are terminated. Using Game.LoadActor() creates a new actor object (which in turn starts its scripts). When you delete the actor, its scripts are deleted as well.
Local variables (declared with "var") are part of the running script. When the script is terminated, its local variables cease to exist.
Global variables (declared with "global") exist all the time, they are not dependent on any scripts (the scripts only use existing global variables or create new ones).
Custom object properties (set with this.SomePropery or actor.SomeProperty) are owned by their parent object. When the object is deleted, so are its custom properties.