That's not what it means though, taking "justice" or "law" into one's hands:
“take the law into your own hands” in English. C2 to do something illegal and often violent in order to punish someone because you know the law will not punish that person: One day, after years of violent abuse from her husband, she took the law into her own hands.
For that situation, it'd be better to say self-defence or defence of others, because it's not about justice or law but about responding with appropriate force.
Alright then. We are likely not agreed. I am not lawyer, so I didn't know the exact definition. It will likely be different in various English countries, I bet;
Self-defence includes defence of others in my country's law. Going away from that example, which was, by the way, investigated as murder, not self-defence.
If a regime forbids you to spread the truth people should be aware of (from your perspection), is it bad to spread the truth? According to your definition, Edward Snowden did bad. According to common sense, well, depends on the person. But I quite admire him and consider it good what he has done.
Another good example. Just re-reading your definition and seeing this "One day, after years of violent abuse from her husband, she took the law into her own hands." If she was regularly abused both physicaly and psychically, I think it is correct she finally stood up for herself. It is against the law, but imagine how long it would take if she announced it to cops. If she was not refused, it would probably take months to get rid of the man. And he would add on his insults --- Of course, the state is probably able to help her in like 3 days, but there is no certainity in that for her. And if she had to face years of humiliation from this man, it is far and justified murder from my standpoint.