If you found their private key by mathematical computation or guessing, I think it will not be considered illegal. But if you got that by using malware or trojan that may be illegal.
If you hacked Nobody can expect you can claim that wallet is yours.
So,
1) If they hacked into my machine and stole the private key; (
illegal)
2) If they sniped the pk while snooping on a public access connection; (
illegal)
3) If they brute forced the private key into existence; (
not illegal)
4) If they snapped a shot of my private key qr code over my shoulder at an ATM. (
not illegal)
What if I claimed an address on the block chain with a signature and 3 or 4 happened? Would I have a case then?