I wonder if they made the combine soldiers as inhuman as possible to make the Combine seem more alien or/and to dehumanize them so the player doesnt feel bad to kill them. I mean, you did see that, despite everythig, the HECU were still human enough so you could still feel bad for them or that they are just tragically "just following orders" and that said orders may have even been manipulated by powerful players like the G-Man. Opposing Force was all about the acceptance of that humanization and showing that, in a way, they were just as lost about this whole thing as the scientists were because their government easily turned on them when it came down to it.
With the combine soldiers, you dont really get any of that. They are always focused on killing you and there is barely any dialogue between them that adds any sort of humanity or warmth to their existence. They are more machine than men, twisted and evil.