What if I found a big Bitcoin address?

Let’s assume I randomly generate private keys and just happen to get the same key as some 1000 BTC whale and getting access to them (I know the odds). Then I transfer them to another address, and go to any exchange to cash out. What would happen? Would I get arrested for theft? Would the officials assume this is illegal money, as I cannot explain where I got them? What about taxes?

Little mind game, what do you think? Of course it’s just hypothetical, as the chance of actually brute forcing such an address is like nonexistent