I think it was simply for the money.

We'll see when Mt. Gox goes up again if it was actually true that only one account was compromised. I have a hard time believing that actually since the hackers obviously had the option to get into more than one. What I find much more likely is that a lot of people will have bitcoins missing from their accounts. The main reason I can think of to crash the market would be that it allows you to withdraw a large amounts of bitcoins from accounts without hitting the $1k limit. I wouldn't be surprised if they managed to empty a lot of Mt Gox accounts of their bitcoins.