Usually, when a trailer outright spoils something as big as the death of a main character or a complete change of direction for an established hero (as with when Terminator: Genisys' trailers revealed that John Connor was in fact a Terminator now) you have to wonder if they've completely lost their heads.
Surely this is the kind of magical story moment that the audience deserves to be kept as a secret? There's a currency to plot, after all, which means major tentpole releases like Avengers: Endgame are locked down to an unprecedented degree. Studios don't usually want us to know. Fans don't usually want to know. So defying that is like flipping the world upside down.
Still, that's exactly what the second X-Men: Dark Phoenix trailer has done. Months after the first was released and with whispers of more delays to release (or an outright cancellation in some cases), the teaser has dropped and it kicks off with the revelation that Mystique is killed.
So why have they done it? Simon Kinberg has the answer as he told EW about it:
"Well, the thought process behind that was to primarily show that this is a movie that is unlike other X-Men movies. It’s a movie where shocking things happen, where intense, dramatic things happen. People don’t just fall off buildings and dust themselves off and walk away. There’s a reality to this movie and a consequence to this movie. Even more than that, it was to show that Jean/Dark Phoenix is genuinely a threat to everyone, including the X-Men."
If you think back to Infinity War's marketing campaign, almost these exact things were said. Kevin Feige and the Russos said there was a major event right at the start of the movie that established that this was unlike any MCU movie to date and that Thanos was indeed the threat he appeared to be. Ultimately that was the deaths of Heimdall and Loki, which Marvel chose not to put front and centre in their marketing.
Why? Because it's not necessary. You can hint at something - you can even say it's the death of a main character - but saying WHO it is is a step beyond that. And there's a reason Fox and Kinberg have chosen to do it. It's a marketing ploy.
Think about it: Dark Phoenix has been beset by confidence issues for months now. It's been shifted back out of its release slots more than once and with the Disney/Fox deal looming, it's being widely considered something of a challenging release (and so is New Mutants, of course).
It's also a bit of a free throw for Fox, since the Disney deal is now their safety net. This movie is probably about damage control on the budget more than anything at this stage. So some drastic marketing techniques were necessary. It wasn't enough to just pretend this is just another X-Men movie, which is precisely why Kinberg is talking about difference.
In the face of a fairly big swell of negativity - which hasn't been abated by the quality of this second trailer, to be honest - Fox have decided to take control of the conversation and guide it somewhere useful. The reason they've killed off Mystique in the marketing is because it's a marketable asset: it's something interesting in an otherwise uninspiring movie (in terms of general audience expectations) that they can weaponise to bring fans in.
By making it seem like Mystique's death is so inconsequential that it can be shown in the marketing, Fox have rather cleverly suggested that there are other things coming in the movie that make that look like nothing. In the same interview, Kinberg mentions other "major casualties" and seems to set up a Civil War style movie where the X-Men split based on how they want to deal with Jean - whether to reform her or kill her - presumably with Beast on one side and Cyclops on the other.
By downplaying one story element, they've not only added intrigue - because we don't know HOW she kills her - but they've also made everything else more interesting. And when the spoiler was already heavily rumoured anyway, it's a bold and clever decision and far better than the approach Paramount took to marketing Star Trek Into Darkness when we all knew Benedict Cumberbatch was Khan no matter how hard they insisted.
This is something of a no lose situation for Fox. Kinberg admits he has no idea whether Marvel will retain the cast (they won't) and nobody is expecting anything of the movie. So why not throw the dice and market it in an entirely different way? It's a little like going into a fight and starting by breaking all of your fingers and shouting "if I do this to myself, imagine what I'll do to YOU!"
Hey, if it works, it works. But it's a bold strategy, Cotton, and we'll have to wait to see if it pays off.