There's a lot more to the Helghast than TEH UBER BADASS METAL SUITS WITH TEH ORANGE EYEZ! Although they are awesome 
The Helghast are definitely inspired by the Nazi Germans. Oppressed for many years, desperate to reclaim their former power, they will do anything.
Also, a little like Japan in WWII. They did a lot of bad things, but despite the fact that it was their enemies who launched the massive attacks (the nuclear bombs, the ISA invasion), they are considered the bad guys.
Guerilla took a lot of information from past civilisations and applied it. If you plan on getting Killzone 2, read this thread on the story so far:
http://www.vgchartz.com/forum/thread.php?id=56896
It's from Killzone.com, just all put conveniently into one page.
(Former) Lead Moderator and (Eternal) VGC Detective 
