Magneto wasn't really an evil villain...

And when he realized he had the opportunity, he planned to take it - He didn't decide in the moment to do that without thinking. It was an action that he devised ahead of time. Whether he devised that action 5 minutes in advance or 5 days in advance, the advanced nature of the scheme makes it, by definition, a plan.

While we don't know exactly when he came up with it, I personally wouldn't put it past him to have had that whole scenario planned out from the moment he escaped from prison after having considered it as a possibility (in a "if I wasn't in here I could use this to wipe out his people" kind of way) while Stryker was pumping him for information. If you watch the scene where Magneto and the X-men join Mystique in the control room you can tell that Magneto already had the whole thing planned out from the tone of his voice when he says, "There it is," and "Come. There's little time," followed by the looks on Magneto and Mystique's faces when Jean responds by saying, "Not without us." Magneto and Mystique then share another conspiratory look while walking with Jean on their way to Cerebro. As a result, he had to have had the plan before meeting up with the X-men as in order for Mystique to have been in on it, they had to have discussed it prior to joining the X-men on the Blackbird. Thus, I think it is fairly evident that he had the plan from that point where Mystique and Magneto were alone in the woods together.

In Psychological Circles, there's NO such thing as "for the hell of it"-There's ALWAYS a reason for the actions of another even if they're not immediately apparent.

One of the scariest things about nazis and a point that almost every work of fiction and non-fiction about them fails to understand is that they thought they were the good guys, protecting european culture and their home. Of course from our perspective they were one of the most unambiguously evil forces in history but to them they were doing something good and necessary. Which is why millions of normal german people decided to follow them and commit the most vile atrocities.
Magneto is similar. To us he is evil, genocidal, megalomaniacal, but if you actually try to understand where he's coming from his point of view becomes quite relatable. This is one of the great strengths of the X-men movies. They are always about clashing ideologies, not about the villain of the week. Even X-men Apocalypse which at first seems to go for that approach is ultimately about clashing ideologies too. The character apocalypse is basically magneto and his ideas turned up to eleven. A crazy deity who thinks he's doing the world a favour by "cleansing" it.