a person who is extremely interested in only one thing, often to such a degree that they are mentally ill

The book portrayed him as a monomaniac and a monster. 这本书将他刻画为一个偏狂和残忍的人。

He was a monomaniac who was not entirely responsible for his actions. 他是一个偏狂者，不能为他的行为负完全责任。

suffering from or relating to monomania (= a condition in which someone is extremely interested in one thing)

The story switches between the neglected genius and the monomaniac academic. 故事在被忽略的天才和偏狂的学者之间跳动。

Monomaniac politicians and media moguls are among the causes of poverty, not their cure. 偏狂的政客和媒体大亨是贫困的根源之一，而不是解决办法。