In almost every instance in which the admonition Don't take the law into your own hands! is used, the intention is that one should defer to some other party. But there are various parties to whom one could defer, some of them rival. A choice to defer at all is itself a choice about what is the law and implicitly about how it should be applied. In choosing to defer to one of these parties, rather than to another, one has already taken the law into one's own hands, if only then to let it go. A person is always responsible for such choices. Sometimes, deference is a very appropriate choice, and perhaps even the only appropriate choice, but one is responsible for choosing when and to whom to defer. The only way that a person could perhaps not at all take the law into his-or-her own hands would be in utter passivity — not even acting to draw some other party into the situation as giver or enforcer of law. And, still, to choose passivity would be a choice, and sometimes a morally unacceptable choice.
Those who insist that we should not take the law into our own hands almost always intend that we should defer to those with the most social power concerning law. Various concerns might motivate that intention, but most often the admonition comes from members of that group (state officials), or from people who take it that the social power somehow arises from virtue of some sort, or from those who believe that the only alternative to deferring to those with the most social power is so obviously barbarism that no argument need be made. If a reader believes that I need to critique any of these cases, then he-or-she should comment below to that effect.