A word for someone reporting about other people to the government

Is there any pejorative word for someone reporting about other people to the government or other power about other people, but who is not working for that power (is no spy)?

That reporting is usually for money, for eliminating rivals, or just because of hate/revange.

Examples:

• reporting oppositionists to the secret police
• reporting sexual/ethnic minority members that hide from prosecution
• reporting that someone is reading forbidden books/watching forbidden movies

Is there a word for such a person? In Poland it's called 'konfident' but in English that word has quite a different meaning.

The translation services give words like 'informer' but they hear quite neutral for me, while Polish 'konfident' is very pejorative.

Your Polish word directs to

Denouncer (latin Delator)

Denunciation - an open public accusation or reporting a person or a group of persons to public authorities, often done anonymously for low political or personal motives, on which the informant has a personal interest or hopes to gain personal benefits.

Informer/Informant is closely related

An informant is a person who provides privileged information about a person or organization to an agency. The term is usually used within the law enforcement world, where they are officially known as confidential or criminal informants, and can often refer pejoratively to the supply of information without the consent of the other parties with the intent of malicious, personal or financial gain. However, the term is used in politics, industry and academia.

Informal synonym: Stool pigeon which I have only seen mentioned in relation to criminal circles.

Grass is an informal synonym to informer and is used mainly in Britain.

informal A police informer.
[perhaps related to the 19th-century rhyming slang grasshopper 'copper']

Usage

replying to a comment and another answer, I would not use the words grass or snitch for a

• Chinese that reports a neighbour for having Falun-Gong books on his bookshelf
• Ugandan that tells the police his male employer has a boyfriend
• Someone informing Gestapo that the priest is hiding Jews in the church.

I would more likely use them for a schoolboy that tells the teacher that one of his bullies has a reefer in his bag or the drug runner that sells out his supplier for leniency in court, which rhymes with the usage mentioned in the comment.

In my opinion snitch/tattletale are synonyms for petty telling on classmates and grass one step up in criminal circles and none of them related to reporting people to the government to get even with them or for personal gain like taking over their house, although such motives can apply to snitches, grasses et al

Another word, in addition to the ones already mentioned, is snitch. The word usually refers to a criminal that reports to the police on other criminals, usually for something in return (money or a lesser sentence, for instance). A drug dealer talking to the police about the people higher up in his organization would be called a snitch, for instance. The OED provides a good example from 1965 of the usage, relating it to the other words mentioned:

The ‘snitches’ and the ‘grassers’ and the ‘stoolpigeons’ whispered out of the corner of their mouths, and money changed hands.

While originating specifically in criminal circumstances, it is also more generally used for anyone betraying a confidence to a higher authority. One could imagine, for instance, that a student that tells a teacher that other students cheated on a test would be called a snitch. Your examples would fit the definition as well.

The word is very perjorative, but it should be noted that in many cases, most people would say that "snitching" is a good thing; that if you are aware of criminal behavior, you should report it. But that would obviously change in a situations like yours, where the police are out to do bad things.

An informal term that is often used is rat, both as a noun and a verb.

(noun) an informer; stool pigeon

and

(verb, intransitive) to act as a stool pigeon; inform (on)

As noted, an alternative is stool pigeon

(informal) a spy or informer, esp. for the police (also ) ˈstoolie

And thence to stoolie