Dursleys under the Cruciatus curse? I guess that would explain their treatment of Harry. The only other theory out there is that him being a horcrux drove them insane, which doesn’t really add up given his friends were fine.
JK Rowling reveals why the Dursleys dislike Harry Potter so much:
https://www.theguardian.com/books/2015/jun/24/jk-rowling-reveals-history-dursleys-harry-potter
Bond of blood charm:
http://harrypotter.wikia.com/wiki/Bond_of_blood_charm