German film about oldman collecting plants who starts cleanly killing neighbors who annoy him.

I’m looking for the name of a movie and probably a German friend could know the answer.

It was a movie broadcasted on Iranian television more than 20 years ago. It was about a retired ordinary oldman living with his wife and collecting pressed plants. I think his name was Mr. Wünsche. One day after his wife nags him continuously, he pushes the ladder while she is cleaning the balcony on top of the ladder and actually he kills his wife. Everyone believes that it was an ordinary incident but a detective feels suspicious about Wünsche.
The story goes on with Wünsche killing two or three more people who annoy him in one way or another. The detective continues to interrogate Wünsche while being accused by his chief that he is mistaken about the oldman.

Whenever Wünsche decides to silence someone new, he puts his middle and index fingers near his mouth as if he is thinking and making a plan.

In the final chapter, Wünsche invites the detective to his house and gives him a poisoned drink and sets the scene as if it was a natural death. A few days later, the detective’s chief comes to Wünsche handing him the print of his photographs from his plants. As Wünsche is watching the photographs he sees a picture of the detective that he has taken from himself as he was suffocating in Wünsche’s house. So the chief arrests him and the film ends with showing Wünsche in prison working on his pressed plants collection.

It was a good film and I wasn’t able to find it later. It was broadcasted with a title close to this: ‘Two-sided War’ which referred to the detective’s struggle with both Wünsche and his chief, but there were many translated titles those days which had nothing to do with the original title.