What is the right course of action now?

In this situation, all three actions—warning the other driver with your horn and lights, moving as far right as possible, and notifying the police—are correct by German traffic law. If another driver is behaving in a way that endangers road safety or causes confusion (for example, incorrectly signaling), you must do everything you can to prevent an accident. Warning the other driver (using horn and lights) is allowed if there is an immediate danger. Moving to the right increases safety by giving more space. Notifying the police is important when serious traffic violations or dangerous behavior occur to help prevent future risks. Knowing when and how to use these measures is key to being a responsible driver under German law.