User blog comment:Miskos3/Fairy Tail Chapters 532 & 533, Review/@comment-5894890-20170422065408/@comment-1776824-20170422150730

Mavis said she had no idea if she could stop Fairy Heart if used and was afraid she'd basically blow up the earth, which is pretty counterproductive to their plan. Zeref also wanted Irene to do it because I imagine he didn't want to be the one to hurt Mavis. He is also fucked up in the head because of the curse of contradiction, so his actions and thoughts may not necessarily align.