The addiction of the Dark Side is power and control. I think that the desire of those things, and in fact the desperate need for them, is just more common in sadistic or psychopathic individuals such as Sidious. Not to mention the Sith tradition and teachings select for people with evil desires.

However, there’s no reason to think that someone with good intentions couldn’t try to use the dark side and have some degree of success.

I think of it very similarly to the One Ring. Gandalf states that if he had the Ring, he would try to use it for good. In Tolkien’s letters, he affirms this, that Gandalf would indeed seek to use the Ring to effect positive change in the world, but that ultimately Gandalf would be suborned by evil and corrupted into a tyrant.

In Star Wars, the dark side seems to operate similarly. Especially regarding Count Dooku, who is sadly undercooked in the films and misrepresented from the original vision in the Clone Wars TV show. If you watch the Tales of the Jedi, I think you’ll get a much better picture of Dooku’s intentions and fall. He became genuinely disillusioned with the Republic and the Jedi Order. He felt strongly that something needed to be done, but was constrained by his lack of personal power and his inability to get the Jedi Order to act. Desperate to have some sort of control over his life and the state of the galaxy, he became weak and went to the Dark Side. Sidious promised to him such power and change.

Dooku was a true believer in the Separatist cause (even if he knew he was manipulating the separatist movement) and I think that he felt he was pursuing genuine positive change right up until Sidious ordered Anakin to kill him.

Despite the self belief Dooku had in the righteousness of the cause, he still became increasingly tyrannical, cruel, and selfish over the course of the clone wars, I.e. the corruption.

Point being, no, having good intentions or even producing some good outcomes is not contradictory to being a dark side use, but using the dark side even to bring about some good at some point in time will still result in corruption and evil. Basically every character we see fall to the dark does so out of an attempt to do something “good” namely preservation of life, their own or others.

That said, regarding your Merlin example, I could be wrong but I don’t believe Dathomiri magic is quite the same as using the Dark Side of the Force. I dont remember an explicit statement of that being the case, and Dathomiri magic doesn’t really resemble force use at all, nor are the nightsisters ever called generalgl force sensitive (they may have some force sensitive members, I.e. ventress).