Killzone in my opinion relates to the US-Mexican history. Mexico as the Helghast, US as ISA. as history tells, conflicts between US and Mexico began in Texas(Vekta), first Mexican territory and now US, than the war started which ended up making Mexico lose more then 70% of its original territory "sold" to the US, history says sold because the US won and therefor the US wrote history but sold is not the truth, we wont know the truth and thats what I think of Killzone and I think the ISA are the bad guys not the helghast who only wanted what was theirs, like Mexico.
If you will recall, before the events of killzone 1 the UCN and ISA actually invaded Vekta and stole it from the helghast. Te Helghan Corp. Actually owned both Vekta and Helghan before the ISA arrived and they even named Vekta after their Leader. In Killzone 1 they were only trying to take back what was rightfully and legally theirs.
Wrong. before killzone 1 the Helghast owned both Vekta and Helghan legally. However the ISA invaded Vekta ( named after the then leader of the Helghan corporation ) and forced the Helghast out. Visari had every right to take his planet back from the ISA invader scum.