Notice the idioms in this letter to the editor of a newspaper

Sir/Madam
During the demonstration in the city center, we saw protesters taking the law into their own hands and attacking the offices of the company responsible for the pollution of our beaches. While I can understand how angry these protesters must feel, it is clear that the most violent group of demonstrators, the so-called ‘Nature’s commandos’, have become a law unto themselves and that the police are failing into control the situation. What we now need is a firm government that is prepared to lay down the law to the big corporations which pollute our environment and to stop bending the rules when it is a question of commercial interests. At the same time, the police must take firm action against the most violent groups. Only in this way we can solve the problems we face now.

Sincerely,
Laura N

Explanation:
Taking the law into their own hands = taking action which should be taken by the police or the courts
Have become a law unto them shelves = are completely out of control and simply ignore the law
Lay down the law = be very firm and clear about the law, and enforce it
Bending the rules = making special exceptions; letting some people disobey the rules/law

Here are some idioms common to use when we talk about Authority and Responsibility
If you:
carry the can
Then you :
accept the blame/responsibility alone, eventhough other people were responsible

If you :
get/let someone off the hook
Then you :
are free/leave someone free from all responsibility or from a difficult situation

If you :
leave someone to their own devices
Then you :
allow them decide how to act; do not control or supervise them

If you :
are at/on the receiving end ( of something, for example, a person’s anger/criticism)
Then you :
suffer from/ are the target of (the other person’s anger/criticism, etc )

If you :
are at someone’s beck and call
Then you :
do everything they tell you to do, whenever they tell you to do it

If you :
get your own way
Then you :
persuade other people to let you do what you want

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