User blog comment:FairyRaven/Fairy Tail Chapter 353: Exorcism, Review and Commentary/@comment-7418318-20130927124923/@comment-7418318-20130927162858

Technically Crime Sorciere is also a Dark Guild from the viewpoint of the mage council, even though they call themselfs an independant guild.

And even if the guild itself is evil, it doesn't mean every single member has to be.

Remember that Flare was also member of a Dark Guild!

Silver might have joined that guild to further some personal goal (like RAVE's Sieghart joining Demon Card for a while) or he might be some battle-junkie with a twisted sense of honor searching for ways to fight the strongest opponents (like Azuma or RAVE's Let), or he might turn out to be another "good guy fallen in with a bad bunch" (like Flare, Juvia, Gajeel, Simon, ect.).

I'm not saying that he's not a bad guy, but for now we have too little information to say much of anything for sure.