Frenchani said what I wanted to say. Ats never really convinced me, and IMO took itself much more too seriously (though they were able from time to time to make fun at this).

I'm not sure I agree with how you sum up the two shows, especially in the case of Ats. It works in the case of Btvs if we think about it in terms of how life's circumstances (for ex living in violent environement...) can reinforce our dark sides and turn us into violent persons or persons who have to fight hard against their own dark impulses. The opposite reverse doesn't work so well: how do you force goodness on someone? Seems a bit contradictory in terms.