Whether you use the darkest of arts in your duelling or whether you use only the less dangerous curses, jinxes and hexes, and whether or not using darker methods is for the greater good, all comes down to your ethics and morals. You can use a dark spell for a good purpose. You may have to use something dark to save humanity. But what saving humanity means to you all comes down to you, for eg. the notorious dark wizard Voldemort thought he was cleansing the world of muggles and mudbloods to protect the reign of the pureblood witches and wizards. The interesting fact here is that he was a half-blood himself. His hatred of muggles perhaps came from his muggle father, who abandoned his witch mother, and left Voldemort an orphan as his mother died at childbirth. Voldemort's muggle father was given a love potion by his witch mother to make him marry his mother, but that clearly had consequences as Voldemort was born with no ability to love. As you all know, love is one of the five exceptions to Gamp's law and cannot be conjured, which is why children born of love potions do not have the ability to love. Love potions create a form of temporary attraction which dies away when the potion wears off, which is how Voldemort's muggle father abandoned his witch mother when she failed to give him the love potion for some time.

You may be faced with an acromantula and need to use a dark spell to protect yourself. But whether you will only use it as a last resort to protect yourself or use it to distract the creature as a means to escape or whether you will actually use it to harm the creature unneccessarily even if it is not required and even if you do not need to harm it to escape, all comes down to you. Whether you consider it to be necessary to harm the creature, also comes down to your ethics and morals.

There are laws by the Ministry of Magic preventing you from harming creatures or humans unnecessarily or using very dark magic associated with the Dark Arts and there are trained aurors in the Ministry of Magic whose sole job is to find and eradicate dark wizards and dark artefacts but there are always loopholes in the system that may lead you to unnecessarily cause harm and that can only be prevented by your own ethics and morals. Remember that violating the laws of the Ministry of Magic can land you in the notorious wizarding prizon of Azkaban which you all know about. As you know, it is guarded by dementors, which cause depression, and if they kiss you, suck your soul out.

However, despite all the laws and rules and regulations, it all ultimately comes down to your ethics and morals.

I would like to discuss something here about the power of love. Love is a very powerful emotion which can influence actions and is magical in itself. The magical power of love is so strong that it can stand against and defeat the darkest of Arts, for eg. when the notorious evil dark wizard Voldemort killed the famous wizard named James Potter and then intended to kill the then infant famous wizard, Harry Potter, Harry's mother's love saved him, as Harry's mother Lily stood in front of her son to protect him which counted as a mother's sacrifice or a sacrifice for love and thus though she was killed by Voldemort, the magic of her love for her son in her son's blood prevented Harry from dying when Voldemort cast the killing curse on Harry and caused the curse to backfire, making Voldemort lose his bodily form, and Harry was left alive, and the only remnant of Voldemort's killing curse that Harry was left with, happened to be a lightning shaped scar on Harry's forehead. This scar was evidence of Harry having become Voldemort's Horcrux, though, accidentally, because of the curse. Voldemort's soul inside Harry as a living horcrux of Voldemort, was eventually destroyed, by Voldemort himself, when he tried to kill Harry again with the killing curse, during the Battle of Hogwarts, and Harry was again left alive with only the fragment of Voldemort's soul inside him dead. I would speculate that Harry could have been saved a second time by his mother's love imbued in his blood. The same love of the famous witch, Narcissa Malfoy, towards her son Draco Malfoy, the arch-nemesis of Harry Potter, also saved Harry once again by preventing Voldemort from knowing that he is alive, thus preventing Voldemort from attempting to kill Harry again, as she wanted to know from Harry the whereabouts of her son, Draco Malfoy.

Love is the magic that can prevent you from turning to and from the harm caused by the Dark Arts. It is a very powerful and potent magic. Also, love is the reason that stops you from using dark magic in the wrong way that causes harm to humanity. Love towards all of humanity protects you from wanting to harm the whole of humanity or humanity in general for whatever reason.

Therefore, there may be reasons to use the Dark Arts and there may be reasons not to, but it ultimately all comes down to you in the end, to determine how your knowledge will play out in your actions.

In the end, it all comes down to your ethics and morals.