Awww, I was expecting all this fuck up is due to humans own greed, ignorance, and stupidity.
But, I guess that reflects better on the OP parents and makes more sense. It’s less that they failed at educating their son and more that their son was misled / fell to temptation by the demon (of course, the hero is not blameless for his actions).
As for the memory wipe, it’s kinda disappointing, but also makes sense in terms of group morale and motivation. If they remember everything the group might disband and/or many of the members will be too depressed to do their mission properly.