User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-24712102-20150824084034/@comment-9595671-20150827234409

I wouldn't rule that out completely, I mean who says that the curse always had to work instantly?

For all we know there could be a version that kills someone slowly over time, and Yury was an unfortunate victim of such. Not all death happens instantly, as long as it can kill its still death magic.