User blog comment:B214/Talk About What You Want! 2/@comment-24173056-20140703135106/@comment-1359322-20140703141315

Like i said, there's purpose is to fight supernatural beings, you can view it as wrong but they are just acting on their believes.

Also the brainwash thing was something mentioned in Volume 7, that was before the Hero Faction was formally introduced. As you can see in Volume 9, all members of the Hero Faction gathered under Cao Cao willingly, because Cao Cao gave them a purpose in life. To combat supernatural beings a heroes.

Remember, from the human's POV, Devils, Fallen Angels, Youkais and Dragons are enemies to the humans, they are viewed as evil beings, even if Issei and the others hasn't done evil deeds, majority of the humans will still view them as evil just under the pretext that they are Devils. And to the Hero Faction, being able to fight against those "evil beings" as heroes are privilege.