This is still very hard to believe, there is no such thing called honor among thieves, how can an hacker go through a lot to hack a security server just to return the money they stole? This doesn't sound right and I believe that we are only hearing half part of the truth
Wasn't the problem for the hacker that they couldn't access the coins securely? I don't know if someone maybe whistle blowed and the other side had some information about them and they made a deal not to disclose the info if the hackers return the coins or something like that. I guess that we only know half the truth is probably right. The hackers might have been surprised themselves that it worked out, overwhelmed by the success they decided to pull it off and then realized that this is going to put them in jail forever if they made a mistake and didn't completely destroy any trace that might lead investigations to them.