User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-7418318-20150323173501/@comment-7418318-20150325211116

And what do you mean by "can't let one person stay bad"?

I seem to remember an entire guild of Demons who stayed bad until their death... or at least the point at which they were last shown. Same for Erigor, the members of Trinity Raven, Jiemma, Kain Hikaru, Rusty Rose, Brain/Zero and still more.

And with the former Oracion Seis members we don't really know if they are good now (except for Hoteye of course). Cooperating with Jellal is in their own best interest so this cooperation by itself does not make them good. For all we know Midnight could just wait for Zeref to be out of the way and then immediately backstab Jellal out of spite for the earlier defeat.