User blog comment:IamJakuhoRaikoben/Chapter 295: Sting and Lector, Review./@comment-4449950-20120817232609/@comment-904888-20120818021329

Can't argue.

If they DID kill the dragon, those two dragons were either pathetically weak compared to other dragon (CA Acnologia, when the entire FT's best mages combined couldn't even scratch it), or else they deliberately allowed themselves to be killed, in which case, they are not 'true dragon slayers'.

It is almost certain that they DO have their respective dragon Lacrimas implanted in them (as seen by their ability to go into Dragon Force at will, something only generation II DS have the ability to do), but how they ended up there I have a more probable theory:

back in the 777/7/7, when the other dragons dissappeared, those two dragons somehow either wanted to die, or was actually dying, and they implanted the Lacrima's into their bodies just before/on the fateful day. Due to this, Sting and Rogue used their 'killing their foster dragon' as a cover story to make them look strong, where as in actual fact their foster parents died was actually a very painful memory they didn't want to remember (especially since they were representing the self-proclaimed-top-Guild)