What we know so far. The surviving Helghast from the near genocide committed by Rico at the end of Killzone 3 have been given asylum back on their traditional home planet of Vekta. Separated by a huge wall, Guerrilla plans to reflect the tense atmosphere of Cold War Berlin.
Frankly, the Helghast become more and more justified in their violence as the series progresses. They get their asses booted off to a radioactive hellhole, so they attempt an invasion to retake their homeworld of Vekta. After they lose the invasion, they are pushed out of the Colonies and back to Helghan. There, their leaders are assassinated when they were meant to be captured, sending the nation into a fanatic craze.
Six months after that, they get nuked by a superweapon, and nearly the entire race is exterminated, with only a few survivors. This genocide ends the war, and as a sign of "goodwill", the ISA throws the survivors into a walled off city in extreme poverty. On their own homeworld, no less.
I think the Higs are well within their right to be pissed off about their situation, and the ISA's continued mishandling of all things military regarding the Helghast, it is becoming harder and harder to justify their actions. The Helghast aren't without their mean streak, nuking their own (abandoned) cities, but they at least have a reason for their violence. They have a purpose. The ISA lacks this motivation, and so they come off as jackasses for the sake of it.
Either way, I'm looking forward to going back to Vekta, and seeing the Helghast in a more politically charged situation rather than a full scale military operation.