Making the villains the super heroes right? Hacking is stealing don't make it look like it's great to be an hacker, the only way to be an hacker and be legal is getting paid trying to hack into projects security and that's after you've agreed to proceed with the team, bounty things you know.
But is it hacking if someone has access to your private keys?
Maybe let's define what is "hack" and what is "access" according to thefreedictionary.com
hack 1 (hăk)
v. hacked, hack·ing, hacks
v.tr.
b. To gain access to (a computer file or network) illegally or without authorization: hacked the firm's personnel database.
ac·cess (ăk′sĕs)
n.
1. A means of approaching, entering, exiting, communicating with, or making use of: a store with easy access.
2. The ability or right to approach, enter, exit, communicate with, or make use of: has access to the restricted area; has access to classified material.
Based on that and your question, accessing my own private keys without any probable and reasonable legal authority or simply because of my own personal authorization can be concluded as hacking. I didn't give any permission to anyone to control any of my belonging, in any way, even if it was because of my own mistake, the private keys are illegally controlled, which it can be defined as hacking.