I repurposed some old Bella content for this fic and thought it worked, enjoy!
Dancia was not looking forward to this in the slightest, Defense against the Dark Arts was either going to be one of the best classes she had ever had or one of the worst with her Aunt in charge, there was no middle ground to be seen there.
Sighing she entered the room with her fellow Ravenclaws and the badgers of Hufflepuff taking her usual seat at Harry's right side while Hermione took the left.
Spreading their notebooks out and readying their fountain pens the trio barely had time to shuffle together when the office door slammed open and their professor made herself known as she muttered down at the open tome in her hand, a moment later she glanced up to meet all of their gazes evenly.
"Hello my wonderful lovely little miniature adults, I am Professor Black and it is a pleasure to meet you all."
Slamming the book she had been perusing shut Bellatrix glanced about the room with open criticism, "You lot are all here for your third year of the Dark Arts, considering the...caliber... of your previous professors I have already decided there is going to be some remedial work involved but no worries, I will be sure to keep it interesting!"
Before she could continue Hannah Abbot tentatively raised her hand but noticeably relaxed when Bella smiled and pointed to her, clearing her throat she gathered her confidence a bit before she asked her question.
"Professor Black, you meant Defense Against the Dark Arts, did you not?"
Bellatrix stared at the blonde a moment before smiling faintly shooting her gaze to the side of the room while quickly coughing into her hand, "Of course dear, obviously."
It took all of Dany's willpower not to role her eyes at the display, yet she managed, if barely.
"Now while we are most definitely going to be doing more practical then theoretical work in this class what I want you lot to understand is what makes a Dark Witch so dangerous. What makes the practitioners of the Dark Arts so terrifying?"
"So with that thought I ask you all, what makes the Dark Arts, The Dark Arts?" Ernie Macmillan raised his hand and Professor Black nodded towards him to reply.
"Well, the Dark Arts are what dark wizards use against light wizards, right?"
Bellatrix hummed at that a moment before replying.
"Yes and no, you see the Dark Arts serve one purpose and one purpose only, to hurt other human beings while either taking or giving something in exchange. While your average light wizard may be able to bring themselves to cast a bone breaker curse and cutting hexes they'll never be able to pull off truly dark spells simply because they do not want their opponent to suffer enough to make it work."
"Magic, in essence is all about visualization and will, if you cast a bone breaker curse you have to imagine your targets bones shattering in graphic detail and you have to want to cause them bodily harm but not to outright kill them, this makes that particular curse useful for aurors and the like."
Bellatrix nodded to herself at her statement as she began twirling her curved wand idly in her left hand while she gestured about with her right, "True dark spells, ones used by Dark Witches and Wizards are incredibly effective in combat. In fact they are so effective that during times of great crisis the Ministry will actually authorize the legal use of them for self defense, the problem is this is largely useless. You see the very act of hurting a fellow human being works against our own innate tribal instincts, we are repulsed at the idea of hurting our fellow witches and wizards and in turn this causes some psychological trauma feedback."
Professor Black began pacing a bit as she continued on, "While this is beyond some of you the crash course on human physiology is that when you are put under extreme stress your body produces chemicals called endorphins, dopamine and adrenaline which essentially make you feel good while numbing out the pain. When you cast a spell fully visualizing and willing it to maim or kill your target your mind can't handle the trauma, so in turn your body releases endorphins, dopamine and adrenaline to take the edge off of what you have just done," Bellatrix sat on her desk while crossing her legs in a ladylike fashion while sweeping her hand across the entire class.
"Essentially the end result is that when you hurt, maim, or kill someone with magic your body makes you feel good in order to cope. So what happens when you realize you get the most wonderful tingly feeling when you shatter a mans rib cage? What happens when you become hot and bothered when you boil a witches blood in her own body and cause her eyes to explode from the pressure? What happens when you experience the most mind blowing orgasm you have ever experienced when you cast the killing curse on a child?"
The room remained silent, no one saying a word, until eventually Dancia raised her hand and all eyes turned to her as she spoke.
"What happens, is you begin to hurt other people because you enjoy it, not because you need to do so."
Bellatrix nodded slowly at that as she turned back to the blackboard, "And that is why the Dark Arts and those that practice them should never be underestimated, because while you may hesitate for moral reasons, the feeling is usually not reciprocated.
"As a caveat, if you ever run into someone who is able to practice the Dark Arts without suffering from those repercussions well...do yourself a favor and don't anger them, you'll live longer and happier that way."
The remaining time of their class was spent in silence as the class took notes off of what Professor Black wrote on the board.