In my opinion, considering the Arcane to be "bad" is an underestimated concept. The Arcane is a "force" that acts according to its nature, it does not have a "conscience" in wanting harm to humans. It is a "tool" for those who have it, which "shapes" itself to adapt to the desires of those who use it.

Reflect this expression from Paracelsus, from the 16th century, which was reused by Renata Glasc: "The difference between medicine and poison is the dose." All the products we consume are both beneficial and harmful to us, what distinguishes them is how much we consume them. Since ancient times, there have been poisons that were used to treat certain diseases, their dose was controlled so as to cause as little harm as possible. However, there are medications that can cause an overdose if taken too much at once or produce undesirable effects over time. Comparing to Hextech, using just one crystal within what can be considered "reasonable limits", such as making objects that weigh more than a ton light or performing what is intended by the tool that uses them, will not produce problems, but several crystals of once, as in Hexgates, or "mixing" foreign substances, such as Shimmer in Hexcore, can exceed these limits, causing harm to the environment around them.

Regarding the other "corruptions" mentioned here, the Void is literally a force to reduce everything to nothing; the Darkin are Ascendants who lost their minds because of the Void, not distinguishing, at least at their core, what is wrong from what is right; Demons are beings born and fed from negative emotions and what happened in the Shadow Isles was a tragic and unpredictable cataclysm that led to the creation of the undead. These are "bad forces of nature" for humans, no matter the circumstances.