From The Wall Street Journal

Up until the crash of 2008, the prevailing view -- called the efficient market hypothesis -- was that the prices of financial instruments accurately reflect all the available information (i.e. the underlying reality). But this is not true. Financial markets don't deal with the current reality, but with the future -- a matter of anticipation, not knowledge. Thus, we must understand financial markets through a new paradigm which recognizes that they always provide a biased view of the future, and that the distortion of prices in financial markets may affect the underlying reality that those prices are supposed to reflect. (I call this feedback mechanism "reflexivity.")

With the help of this new paradigm, the poisonous nature of CDS can be demonstrated in a three-step argument. The first step is to acknowledge that being long and selling short in the stock market has an asymmetric risk/reward profile. Losing on a long position reduces one's risk exposure, while losing on a short position increases it. As a result, one can be more patient being long and wrong than being short and wrong. This asymmetry discourages short-selling.

The second step is to recognize that the CDS market offers a convenient way of shorting bonds, but the risk/reward asymmetry works in the opposite way. Going short on bonds by buying a CDS contract carries limited risk but almost unlimited profit potential. By contrast, selling CDS offers limited profits but practically unlimited risks. This asymmetry encourages speculating on the short side, which in turn exerts a downward pressure on the underlying bonds. The negative effect is reinforced by the fact that CDS are tradable and therefore tend to be priced as warrants, which can be sold at anytime, not as options, which would require an actual default to be cashed in. People buy them not because they expect an eventual default, but because they expect the CDS to appreciate in response to adverse developments.

AIG thought it was selling insurance on bonds, and as such, they considered CDS outrageously overpriced. In fact, it was selling bear-market warrants and it severely underestimated the risk.