There are hacks that are not illegal. For that, we can go down to kinds of hackers.
There is this..
White Hat
Enter the White Hat hacker, the good guy who uses his (or her) capabilities to damage your organization — but only hypothetically. Instead, the real purpose is to uncover security failings in your system in order to help you safeguard your business from the dangerous hackers.
The Black Hat
What is a black hat hacker?
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A black hat hacker is typically one that engages in cybercrime operations and uses hacking for financial gain, cyber espionage purposes or other malicious motives, like implanting malware into computer systems.
And the one that traverses in between
The Gray Hats
Some Gray Hat hackers like to believe they're doing something good for companies by hacking their websites and invading their networks without permission, but company owners rarely appreciate unauthorized forays into their business information infrastructure.
In most cases, a Gray Hat's real intention is to show off his skills and gain publicity — maybe even appreciation — for what he considers a contribution to cybersecurity.
White hat hackers have consented to hack the server of a company that hire them to look for holes and glitches in the program used in their service. This kind of hacking is legal because of the agreement of the owner and the hacker.
But can be a private key illegally controled? Do you own the coins or do you own a key to access the ledger? This is my question, remember the hack against the pancakeswap admin, maybe not the best example, but still
If you gain your access thru illegal means, the next steps will definitely illegal. You cannot say what you did is legal when you did get your access illegally, (please refer to the given definition of hack). Unless you have the consent of the owner of the privatekey.