It's not just that there is satanic black magic, but that the satanic black magic is consistently portrayed as a good thing. Geralt really isn't any more virtuous than the monsters he fights. Some segments of the game were pretty good, from the opening up through the bloody baron, only for the game to slow to an exposition-crawl in novigrad. The last third of the game where you go after all the main villains is very rewarding, but that huge middle portion of the game is rough. I will also say that the first dlc was very interesting, the second was so-so.
The first DLC was incredible, me and my wife thoroughly enjoyed it as it was about the 'mirror' and the adversary was the representation of the devil. I think you might've misunderstood the game a little as most of the gems in the game overall were in the side quests.