Not everyone likes to spend time in jail. Some people would say, give me millions and spend me in jail for ten years but some people just simply can spend a week in jail, it would make them insane and it also depends where the person has to be in.
Maybe all he needed was to become popular for a second and that's all. Otherwise, if he is a white hat, we would just reveal the bug. And if he is good at anonimity, he would keep the gains (millions) in his pocket. Seems he was able to hack but unable to hide his identity.
According to the statements made by the hacker he was afraid that if he reveled the bug to the developers there is the possibility that one of those that got that information could have used the bug to get the money instead and disappear with it.
And I think the hacker has a point since we are talking about 600 million dollars here and that is too much money to not be tempted by it, but obviously that only makes sense if we actually believe his story, which many decide not to and think he simply gave back the money as he was afraid to go to jail.
