Lemming42
Arcane
The atmosphere theft plot had two versions, I think - one where the Earth was being terraformed to make it a new world for Advisors (and toxic to humans as a side effect), and one where they were pulling Earth's atmosphere through a big teleporter to power some weird turbines on their own world.
The Combine do seem to have broadly made sense at some point in development - they're a collection of conquered alien worlds who take the genetic material of enslaved races across their empire and stitch them together into weird new shit, and they've come to add Earth to their empire. Their invasion of Earth is motivated by the aforementioned atmosphere-theft rationales, plus the potential to take humans and make them into Combine creatures, plus free labour (the cut child slave factories, for example). Then there's whatever's going on with The Consul, where he's voluntarily turning into a cybernetic sperm or whatever.
Very little of that's left in the final game of course which I think is what makes them feel so weird as antagonists - they don't appear to even be getting anything from the occupation of Earth in the released game, and yet they're still putting substantial resources into it. You could say that they just want human genetic material for its own sake, like how the Borg will assimilate just about anyone, but then they've put up a suppression field that actively prevents new humans being born. Which will leave them with a dead world in one generation, for which they seemingly have no future plans. The only thing they really do is take humans and modify them in rather unimpressive ways at Nova Prospekt, but then almost all of those humans seem to be tasked with guarding Earth itself.
The Combine do seem to have broadly made sense at some point in development - they're a collection of conquered alien worlds who take the genetic material of enslaved races across their empire and stitch them together into weird new shit, and they've come to add Earth to their empire. Their invasion of Earth is motivated by the aforementioned atmosphere-theft rationales, plus the potential to take humans and make them into Combine creatures, plus free labour (the cut child slave factories, for example). Then there's whatever's going on with The Consul, where he's voluntarily turning into a cybernetic sperm or whatever.
Very little of that's left in the final game of course which I think is what makes them feel so weird as antagonists - they don't appear to even be getting anything from the occupation of Earth in the released game, and yet they're still putting substantial resources into it. You could say that they just want human genetic material for its own sake, like how the Borg will assimilate just about anyone, but then they've put up a suppression field that actively prevents new humans being born. Which will leave them with a dead world in one generation, for which they seemingly have no future plans. The only thing they really do is take humans and modify them in rather unimpressive ways at Nova Prospekt, but then almost all of those humans seem to be tasked with guarding Earth itself.