What was dark about Naruto, really? Madara never killed any characters :| people hype him to be some kind of god yet he couldn't kill one kage, neither did Sasuke or Naruto kill anyone, the series was never dark to begin with.
Well, he couldn't anticipate Tsunade surviving bifurcation. He wiped the floor with five Kage alone and had Tsunade not been able to summon Katsuyu, they'd all be dead.
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The series is dark, and dark doesn't just mean murdering. Naruto was dark in a more mature and subtle way. From around the Kage Summit Arc, everything took an especially bleak and heavier tone. I watched Part 1 enjoying the fights and adventures because they were light-hearted and easygoing, and even the sadder parts didn't drop the tone too low. But by the time of the War Arc, everything was heavier and even the most entertaining fights were tinged with gloom.
OT: I don't think it's worrying. The latter arcs of Naruto were heavy, and I think the lighter tone will make a nice lift from that.
