I'm beginning to suspect that something is rotten on the planet of Vekta.
I'm not terribly familiar with the events of the Killzone trilogy but as I understand it, the evil Helghast - subtle name there, by the way - started some trouble and things were looking pretty dicey for awhile but then somebody dropped the hammer on them and that was pretty much that. In Killzone: Shadow Fall, set 30 years down the road from Killzone 3, the Helghast homeworld remains a blasted wasteland and refugees from the planet are resettling on Vekta, segregated from the native population by a giant wall.
That's naturally a bit of a tense situation, as some people apparently don't like the idea of living next door to other people who not so long ago were trying to wipe them from existence - and some of those people might possibly want to take advantage of those tensions to further their own aims, and perhaps push that understandably-jumpy populace into doing something rash and regrettable to the newcomers. "The right to defend ourselves against foreign aggressors, by whatever means necessary" does sound a tad ominous, after all, especially when it's delivered in that special, "I am crazy and looking for trouble" tone.
Killzone: Shadow Fall comes out on November 15, exclusively for the PlayStation 4.