When people have little incentive to behave well, and when nobody is watching, what do people do?  The last few years have given us millions of opportunities to answer that question as people living in foreclosed homes decided whether to leave those homes in decent condition or to instead pour concrete down the drain.  
The option chosen by about half of those people?  

Here’s a story from ABC News
Myra Beams, a realtor in Tamarac, Fla., said half of her foreclosed properties, regardless of the price range, have been vandalized by the former owners. “I think the former owners are angry, and for some reason, they think they’re entitled to destroy properties,” said Beams. “I guess they’re angry at the banks for giving them the mortgage.”
From the New York Times, on whether it was a good time to shop for foreclosed homes:
“The pour-concrete-down-the-toilet trick is one that most good [home] inspectors know to look for.”
From Zillow
…Baseboards, appliances and a brick pathway completely removed.…Cement poured down toilets…..Then, there’s the one about a slew of dead fish left to rot in an attic.
And from the WSJ
…embittered homeowners have stripped out appliances, punched holes in walls, dumped paint on carpets and, as a parting gift, locked their pets inside to wreak further havoc. Real-estate agents estimate that about half of foreclosed properties to be sold by mortgage companies nationwide have “substantial” damage…
People have strong moral intuitions about revenge, so this behavior isn’t surprising.  
Larry Summers famously said that the most important lesson of economics is that no one has ever washed a rental car.  An excellent point, but the problem is worse than that: Without an incentive to preserve, many of us will gleefully destroy.  
Coda: Sharpie parties“, presumably kids breaking into foreclosed homes.