I think that there is something there with the unnature of what's happening with the children. I can't stick words on it yet. Something to do with the inside-out consumption of the necromorphs. It's vey '80's sci-fi,' where we are the enemy or the the enemy is inside us. 80's versions of The Fly, and Invasion of the Body Snatchers come to mind. The Unitarians seem like this too, they aren't an enemy with defined borders we can fight, they're right here among us. Some combination of the fear associated with the cold war, and the fear associated with 00's era terrorism. Will think more on this.
I'm going to add a 'symbols' section to the page. I was originally going to attach a blurb to The Pack under the 'new enemies' section, but it got too bloated.
The Pack & Crawlers are an expansion of the consumerist theme that ran through the 1st game. Planet cracking is just a way of symbolizing one generation eating up all the resources and not leaving any for the next. So, the next needs to planet crack somewhere else, and so on. Well, these two enemies (The Pack specifically) show up in a mall area on the moon Titan. The mall being another symbol of consumption, and Titan being the first cracked thing; Titan was cracked because of a lack of resources, and that lead to an expansion of consumption.
That's it in a nutshell. Throw some other symbols at me, and I'll include them as I revise.