Unironically despite the get-up, the Helgahst have an interesting backstory as it was written by a team from the Netherlands that knew there was more to WW2 than just 'Germany Evil'

They colonised Vekta and made money off the resources of Helgan, when they wanted independence from Earth and the ISA by extention, the ISA attacked resulting in the first intergalactic War that the Helgahst lost and were forced off Vekta to Helgan which was inhospitable to human life. They were expected not to cope and submit to the ISA, instead they mutated due to the planets ecosystem and created the Helgahst.

There's more but in the first game and following titles, there's a lot of emphasis that the bad blood caused by the first War is the reason why Helgahst consider victory at ANY COST a legitimate action.