If the point of playing The Sims 2 was to kill off all of your Sims, then you would be the world champion! But, unfortunately, the way things stand now, The Sims 2 is still a LIFE simulator. You could actually exit the lot without saving and maybe try this household again. Or, if you were trying to kill your Sims, and we aren't saying that you were, then feel free to move another family into this lot.
http://www.geocities.com/d00med_2_die/pix/sims2/all_dead_msg.html