User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-7418318-20150323180532/@comment-82.20.156.238-20150324082640

Its possible that Silver didn't know the side effects of the Demon Slaying Magic assuming that's what's happening to Gray. It could be because Silver actually learnt the magic whereas Gray just had it given to him and could not properly learn to use it. Or perhaps its a side effect of Gray taking the full brunt of Memento Mori. I like the Mr. Cursey theory though. Whatever it is, I'm sure we'll find out soon. Oh, and I really don't think Silver is/was evil.