User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-5444132-20130329113601/@comment-7418318-20130329123742

If he really is evil, that still doesn't mean he's lying about his objective.

Remember that the "evil" Dragons want to wipe out or subjugate mankind. And between the losses they took from the Dragon Slayers and the Slayer's betrayal of the Dragons they were allied with, the Dragons have plenty of reason to want all Dragon Slayers dead, including Rogue.

So even if Rogues ultimate objective is something different, killing all those Dragons would still be in his best interrest, if only for self preservation.