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Perhaps a good hacker trying to prove a vulnerability in the system?
They would have acted differently to begin with. You dont have to steal all the coins in order to proof a vulnerability.
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Guys, what's your take on this? What do you think that stirred his emotions to return the coins? Probably this could be used on other hackers to return stolen coins too.
Maybe there was undisclosed pressure on the attcker. Maybe they realised that its more difficult to safely redeem the coins than they had imagined.