Spoiler:
So either someone impersonated Dumbledore (which would look a lot, to Luscious, as if Dumbledore told him in private that he's killed his wife, and in public then denied it), or Dumbledore killed Luscious' wife.
Both make sense in the sense of "the bad guys are much tougher". Dumbledore being the real big-bad means that the big-bad won the last battle. The big-bad impersonating Dumbledore means that he managed to take someone with only tangental support (Luscious) and turn him into a weapon aimed strait at his biggest enemy. Both are an "upgrade" over the Harry Potter standard opponents.
Both make sense in the sense of "the bad guys are much tougher". Dumbledore being the real big-bad means that the big-bad won the last battle. The big-bad impersonating Dumbledore means that he managed to take someone with only tangental support (Luscious) and turn him into a weapon aimed strait at his biggest enemy. Both are an "upgrade" over the Harry Potter standard opponents.

