As book lovers, we say this when a book upsets us somehow. It emotionally ruins us. Maybe the reader got attached to a character that dies, or something unexpected happens in the book and the reader becomes emotionally troubled by it. This is a very common expression for book lovers to use. In other contexts, it can many different meanings, but in this context, it means to upset the composure of a person. Here is one dictionary entry for this phrase in case you would like to familiarize yourself with its other uses: https://www.merriam-webster.com/dictionary/undo I hope this helps :)

就是看的好好的，突然來個爛結局這樣的心情吧

Here is the full context: "I am still reeling from that final unexpected blow. This novel totally undid me."

That’s specific to the person, so I don’t know for sure. When I read that line I think “undid” means “to question what he believes”. So the novel made him question what he believes ((whatever the topic was)