On the other hand, this guy probably helped to make the bubble burst sooner -- and thus before the banks could make even more sub-prime mortgage loans -- and that would have the effect of reducing the banks' ultimate losses (and resulting in them applying less of a risk premium to mortgages).
Trying to figure out which of these effects is larger would be pure speculation.
One concern was that even if Mr. Paulson bet right, he would find it hard to cash out his bets because many were in markets with limited trading. This hasn't been a problem, however, thanks to the wrong bet of some big banks and Wall Street firms. To hedge their holdings of mortgage securities, they've scrambled to buy debt protection, which sometimes means buying what Mr. Paulson already held.
In other words, Paulson was buying mortgage insurance contracts, driving their prices up, and now banks are buying those contracts from him (at higher prices than they would have if he hadn't been involved in the market).
By all rights, his initial foray into buying the swaps most likely had minimal effect on the price. Ultimately he felt they were incorrectly priced by the market and made a bet on them.