I never understand axeing it on the last arc, you dus create a lot of hateful fans and lose credibility, why not finished it properly you can see the finish line.
One in the past one in the past and present the "humans" have killed in the past and present so they are slightly more evil then the new demon army who was more evil in the past but is now run by basically a good lich.
Arthur has a position of power in the capital, killing her might advance their own position of power. Doesn't matter how "harmless" she is she is she still could use that position of power and be a threat to someone.