What about the Advisors and the rest of the Xen alien race that has invaded and that are killing the humans?
Do we need another game to show that we can clobber more headcrabs, blow up floating slugs, and suffocate a bunch of leeches? No, unless you really want Animal Extintion Simulator 3.
I'm sorry, but this is my opinion. We don't need a game to tell a predictable story. Unless valve wants to make a couple bucks, then by all means go for it.