I think I have found the novel I was looking for. It's not a 100% about what I said in the OP, but very close to it.
In Hungary, we have a famous kids' novel called A Pál utcai fiúk (The Boys from Paul Street). It is set in the 1920's, and it's about two gang of boys, who play a war to get a piece of ground, which they can call their own. On one side are the Pál utcaiak (the "Paul Street Gang") and on the other side are the Red Shirts. At the end, the Paul Street Boys win, but one of them dies, and the ground will be used for construction.
BUT, the novel in question is a sequel to this. The original was written by Ferenc Molnár in the beginning of the XX. century; while this "sequel" was written by a journalist called Zsolt Kácsor in 2012.
The novel is called Rettenetes Vlagyimir (Terrible Vladimir), and it is set in the 1950's where there was a communist soviet-style dictatorship in Hungary. So the boys from the first book are adults here, they are in The Party, they call each other comrade, they have a Great Leader, so everything is how I wanted to.
The 3 main characters are Wendauer and Szebenics (both from the former Red Shirts), who are two "cops" or thugs for the system, and the third one is their big, black car (it is called Terrible Vladimir by their victims).
The book makes a wonderful work by putting the beloved characters into a position where they soon become repulsive, and also it is a horrible document from the '50's, of the meaninglessness and futility of the soviet style regime.
Only 194 pages long, I really enjoyed it, and read it in 2 days. Hope I can find more like this in the future!
Sadly it is only available in Hungarian, and you have to be Hungarian to understand all of it, I think…