When the game designers created the Killzone universe, they looked at their planet and it's history for inspiration.
Germany felt as though it was under constant attack from the West before WWII, and they deeply resented the Treaty of Versailles. The way that Germany felt could be mirrored by the Helghast and there resentment toward other human colonies due to the way that they were being treated.
Germany got hit hard by the economic depression, it was also destyoed by WWI, other events such as the invasion of the Ruhr, hyperinflation, political uprisings from across the spectrum, the country was falling apart, and living standards were dropping fast. Helghan isn't exactly the nicest planet in the galaxy to live in.
Visari shows many parallels to Hitler in the way he gives speeches, his aims, and his way of leading. The speechs at the start of Killzone and Killzone 2 shows that the used similar styles of propaganda to Hitler.
The way the Helghast army present themselves is also incredibly similar to the way the German army did so. The German army often wore superlative uniforms and was overly-cautious about presentation and showing order.
So, yeah, I pretty much agree with your conclusion about the inspirations for Killzone.
Why, then does everybody have Helghast avatars and signatures? Well, because they look so damned awesome, is why.