The worst couple that I have read about in a book is probably Mr.and Mrs. Twit, from Roald Dahl's The Twits .

I think this because, (it's obvious if you've read the book) their sole purpose in life is to make their partner’s life hell, yes that's right, and they've been married for a longtime.

First they started pulling harmless jokes on each other, to make the other one feel silly, but that soon developed into more. Soon Mr. Twit was tying his wife to balloons and letting her go, to float away forever.

Luckily this plan backfired on him, I say luckily because without Mrs. Twit, the story would not have been complete.