As the world ends, we find out just what kind of man Magneto is.
Every economist, philosopher, and Ayn Rand enthusiast will endlessly debate the color of the sky, but they can usually agree on one thing. People respond to incentives. It doesn't matter if they're Donald Trump or the Dali Llama. If the incentives are right, they'll usually follow what's in their best interests. To do otherwise would be like running a red light during rush hour. It's just plain stupid and helps nobody's agenda.
Every now and then, the incentives will align themselves in a way that makes even the most unrepentant villain into a hero. Sometimes those incentives have to be pretty extensive. If someone like Lex Luthor and Thanos are going to save kittens from trees, they need a very good reason and one that serves their typical villainous interests.
For someone like Magneto, however, the incentives don't have to be extensive. He's still not the kind of guy who can be bribed or sweet-talked into being a hero, but he does tend to be more reasonable than most. That's a big part of what makes him an interesting and likable character, although his likability is often limited by how well the Michael Fassbender's and Ian McKellen's of the world can portray.
Throughout Cullen Bunn's exploration of Magneto in the post "Avengers vs. X-Men" era (or the current Fox vs. Marvel era as it were), he's forged this villain/anti-hero persona who navigates a vast gray area between being a semi-hero and being a jerk. Magneto will do things that even Wolverine will hesitate to do on his worst day, but he'll do it for the right reasons. And it's because he's entrenched in this gray area that the narrative of Magneto #21 is so compelling.
The world is ending. "Secret Wars" is about to begin and the only one making a concerted effort to stop it at this point is Magneto. It's a rare situation where all the right incentives are in place. The Avengers, the X-Men, the Fantastic Four, and Squirrel Girl have failed. There's nobody left to stop the final incursion. Magneto, however, is in a position to do something about it. Sure, it requires him to usurp power from Polaris his only non-retconned daughter, but he's willing to do that and not apologize for it. That's what makes him Magneto.
It's that unapologetic, I'll-do-what-I-need-to-do-and-be-as-mean-as-I-need-to-be attitude that highlights the best and worst of Magneto. Throughout the course of this story, it's just him staring down the oncoming incursion the same way a cow stares at an oncoming train. As he's doing this, others including Polaris are watching him. And they're hoping he succeeds. These are the same people who he once terrified as the mutant equivalent of Jason Vorhees. It's a strange situation for both sides, but one that feels oddly appropriate for the situation.
This sentiment is reinforced by a series of flashback scenes that remind readers that Magneto is definitively not a hero. Cullen Bunn goes out of his way to make this abundantly clear so that there's no ambiguity. This is still the man who tried to launch a nuclear attack against the human race. This is still the man who ripped the adamantium out of Wolverine's body and probably enjoyed every second of it. This is a man for whom the cries of his enemies are like the opening chords to "Stairway to Heaven". And yet, he's also a man who will save the world when called upon.
It's the ultimate culmination of the journey that Cullen Bunn has given Magneto over the course of this series. It's a journey that has kept Magneto in this nebulous gray area between being the kind of guy who rubs elbows with Cyclops and being the kind of guy who strangles Cyclops in his dreams. In the end, even as he's trying to save the world, Magneto remains in this area. He never tries to go full-villain or full-hero either. Like Lady Gaga's fashion sense, he just does things his way.
His methods are cruel. His personality is harsher than a Russian winter. But at the end of the day, Magneto still wants to create a world for his people where they don't have to endure the cruelty that he did. It's a goal that allowed him to be both a friend and an enemy of the X-Men. It's also a goal that put him in a position to be the only one who has a chance at stopping the final incursion. Like Gordon Gecko, he's as good as he needs to be and not a fraction more.
There's plenty of depth on Magneto's motivations. There's plenty of details on why he does what he does. However, some of those details are a bit narrow in that they don't focus much on Polaris or his former children in the Maximoff twins. And no amount of depth and detail can change the fact that he spends the entire issue just hovering in mid-air playing a game of chicken with an incursion.
As such, this is an issue that needs to be taken within the context of the entire series. On it's own, it's just an issue of Magneto having no regrets for anything he did, even as he's saving the world. That in and of itself is still a pretty compelling story, but it's a story that doesn't have the same impact without the cumulative impact from the previous issues. Like someone who has only seen the second half of the Matrix, it's going to feel incomplete and confusing at times.
In the end, Magneto can't stop the final incursion. However, Magneto #21 details his struggle and puts it in just the right context. There's no ambiguity as to why Magneto is doing what he's doing. There's no final realization or change of heart either. To his dying breath, he's still Magneto and he still makes no apologies for anything he's done. He might not be the kind of guy anyone would hire to babysit their kids. However, Cullen Bunn's run on this series has proven that this is the kind of guy you want on your side.