Before I begin… never did I say the series doesn't have "dark" elements, but it still isn't a dark series at its core.
@Mex:
Ehm… They took children and rised them as weapons... What may they gain from their Master death... Mhmmm... But as you said some of them would gain something and others would not.
Doflamingo doesn't need to die for characters like Baby 5 to escape Doflamingo. It's already happened!
Marineford: hundreds or thousands of deaths. Not counting the Traumatize For Life Your Little Brother moment.
Yes, this was pre timeskip, and shows that the series hasn't "changed" after the timeskip. It's still got the same amount mixed in.
Punk Hazard: A man got stabbed to death by Kinemon and they did not even land on the island yet… (Others implied to be dead before) Vergo and Monet may be dead. Plus they were using children for experiments with mortal drugs...
Kin'emon cut down the people on Punk Hazard presumably because they attacked him first. Did they actually die though? Who knows! Monet and Vergo are very much alive, as their deaths add nothing to the story. The Punk Hazard kid stuff is pretty dark, yeah.
DD killing people for free all the way.
Crocodile killed people too. Enel killed people too. Lucci killed people too. Villains have killed people throughout the story.
Law's flashback, genocide to rise big money. Even Ohara bustercall was not that despicable (They were protecting "their point of view peace" hiding the past history).
Oh man… the Ohara Buster Call is not defendable, sorry. Any genocide is despicable and not justifiable by any means.
Dressrosa in chaos, people dying all around, a baby girl was holding his dead father… Not so dark, you are right.
Alabasta in turmoil, Skypiea being destroyed with evacuating children and old Shandians, Marineford in chaos (with a crying Coby noting people dying left and right). Same amount of stuff spread around throughout the series.
–- Update From New Post Merge ---
@TheGreatestSwordsman:
I've already written down that Oda could have easily kept Whitebeard alive and had a very similar effect that we've had so far. Whitebeard after the war is so sick he can barely move, and due to such actions, Blackbeard has taken advantage of his illness and taken all of his lands. Since Whitebeard can no longer fight, the Whitebeard pirates have to flee and here is the same story with a new Yonko. Add Blackbeard being able to steal abilities despite them being alive and the same result applies. Even for Ace, Luffy post-timeskip is the same. You can substitute situations with characters dying and living to the narrative and it depends on readers with what they make of it.
Tell me in this scenario what is different. What changes so much that makes the situation with Whitebeard that he has to die?
So uhh, you realize that Whitebeard died to symbolize the end of an era, right?
Yes, Luffy is the exact same. The crew would've went straight into the New World without Ace's death and been decimated. Ace's death made Luffy realize that he and his crew needed to become stronger to face what lied ahead.