worst case:
entire gulf will become dead zone, gulf of oil will flow into atlantic, eventually effect europe...southern US fishing and tourist industry collapse, fumes and oiled beaches will result in many homeless, unemployed, effect on economy will be significant, fuel and food prices go up, food/produce products from southern states may become contaminated, oil rain and oil hurricanes, sinking sea floor creates tsunami that hits all the gulf states and disturbs new madrid, yellowstone... massive exodus,
(provoked by toxic fume deaths, or any of the many other possibilities) could result in martial law...