Read more: https://blog.coinspectator.com/2017/07/04/billions-stolen-from-south-korean-bitcoin-exchange/
It's funny how every single exchange claim that they are the number one most secure bitcoin exchange but really every single time there is a hack happening that the users have to take money out of their own pockets to compensate for.
This is a major loss indeed and it would be interesting what the hacker does to this money. I think it is unlikely that he will sell off the bitcoins at all because that means potentially having the coins confiscated by law enforcement, so i don't think that we need to panick this much about the prospect of the hacker dumping the coins, driving the price of bitcoin down.
Exchanges should make a clear in an event of a hack, who will be responsible and how will users be compensated. A lot of exchanges don't even have that in their terms and conditions leaving them to do whatever they want.