User blog comment:B214/Volume 21 info & spoilers/@comment-82.199.219.5-20160326131457/@comment-27232228-20160327221737

@swampydrake I don't really think of them as a criminal, though. And since this is Ishibumi we're talking about (the background of the mythological beings featured are pretty much closer to the real myths but the characterization is all DxD original), I speculate that the Malebranche are powerful Devils who is just doing their job, but the Old Families fear them and decided to seal them due to some unspecified conflict (they possibly know something about the Underworld that our heroes don't).