Is it just me, or does the UAC seems to get more evil with each installment?
In the original storyline from Doom, sure, the UAC opens a portal to Hell, but it's a result of carelessness (or hubris if you go into a Prometheus-type interpretation) - I'm pretty sure they didn't intend to unleash Hell, both figuratively and literally. And since non-UAC personnel continue to screw up post-Doom II, the UAC don't seem like such bad apples.
Next you have the context of Doom 3, with Betruger willingly working with Hell. That's a bit more of a blatant villain, but at least in the Doom 3 novels, it shows that there's plenty of honest people within the UAC. Earth being in the sore state it is in the Doom 3 continuity, there's plenty of UAC personnel trying to find a solution to the problems Earth faces (problems left unresolved because...well, I dunno, the story just stopped).
Then you have the Doom movie, where the UAC is quite happy to sanction gene therapy on non-consenting individuals. Criminals, sure, but here, the hell (only figuarative this time) the UAC unleashes is entirely its own doing.
And now we have the Doom reboot, with the UAC entering full cult status, or at least its Mars base. Huh. Did the rift they find lead to the Warp or something?
On the plus side, it does raise some interesting questions, such as how isolated Mars is in this continuity (i.e. in Doom 3 it took 36 hours to get from Earth to Mars), and if they're so isolated it allows them to 'listen to the devil' and even separate themselves societally. And the codex idea is a nice one - liked it in Diablo III, could work here as well.
And this is the point where I'm told to stop talking about story in a Doom game, so I may as well shut up now.