Agreed. It is not that it is dark, but that the beginning of it is hard to avoid, and the alternative to it actually starts the same way that the DB starts -- with a traumatized kid who has gone cukoo and the killing of another cukoo sadistic child-abusing old lady.
Actually, for both questlines, the good and the evil, this is a completely implausible starting point. I mean, really, who would actually go kill a person because an obviously crazy kid is doing crazy rituals?
What my characters would actually do, is get the kid help, talk to the Jarls of Windhelm and Riften, investigate the old lady and have her sent to prison, so a better orphanage keeper could be found. It does not make for good gameplay, perhaps, but that is a much more likely course of events.
The Oblivion DB start was much more plausible, and also made the choice to seek out the DB an actual choice -- after the DB spotted you and gave the invite, if you wanted to go there, you had to actually go there. You did not get railroaded into it by a highly preposterous yet difficult to avoid jump-off point. In Skyrim, I really have to be meta-gaming to even pursue the quest further -- I would only go forward with the storyline because I want to get more quests and storyline. My actual characters, good or evil, would not see this crazy kid as having any real way to move toward the DB, either to join them or to destroy them.