But of course, you can have dogs of any breed which are mentally ill. We had a GSD that proved to have what was apparently a brain tumour. He would suddenly stare at you. If you shouted at him when he did that, that seemed to bring him out of it, to distract him. If you didn't notice, he would rush across and attack you. Once you fought him off he would slink away and sit shaking in the corner, looking totally bewildered.
And , many years ago, my parents a golden retriever that was psychotic about cats. She never chased them. She would lie peacefully, the cat or kitten would slink up and rub itself against her, purring , this would go on for a minute or two, then she'd swiftly grab it and kill it.