User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-104.7.80.176-20150719200005/@comment-104.7.80.176-20150719200443

Because he's a dick and probably wanted to do issei the exact same thing he did to vali. Turn his parents on him and then kill them just fun and to alleviate his own boredom. P.s. don't know if he killed vali's mom but he definitely kill vali's dad his own son.