The thread was about destroying a civilization, not a world, so I'll add in a pretty good way to destroy a civilization:
A political faction that supports big business and the rich only begins to lower politics to the third grade level by reducing everything to jingoism, name calling, insults, etc. They do this through talking heads who do nothing but spew childish insults while telling lies and half truths.
The faction accuses anyone who disagrees with them on any issue of being unpatriotic, socialist, communist, soft on crime, etc.
When in power, the faction passes one law after another that benefits the wealthy at the expense of everyone else. They dismantle regulations meant to control big business, protect the public from monopolies and exploitation, protect the environment from ruin, protect consumers from being cheated, protect workers from being abused, etc. Anyone protesting these moves is simply dismissed as a "socialist" or some other negative buzzword.
As the rich get richer while the median income drops, the faction tells everyone that "average" incomes are rising.
Allow employers to ship jobs out of the polity to other polities that use cheap labor such as child labor, prison labor, etc. and that ignore environmental protection laws.
The rich continue to get richer, the rest of the populations income, standards of living, etc decline. Meanwhile, try to start a bogus war that allows you to take the tax money of the majority and give it directly to corporations under the guise of funding the war.
Let the rich use external tax shelters to keep from paying taxes. Meanwhile the tax revenue of the rest of the people drops as they lose their jobs and incomes drop. Continue to take the remaining tax money and funnel it to major corporations through the bogus war.
Eventually the middle class collapses and the government has no money to run things as the middle class don't have any money to tax and the rich aren't paying any. Then the civilization implodes.
Works great. Just look at real life for examples.
(or maybe the recent Firefox update is a bit buggy...)
