Given that we now know that technically, the Doctor was on his 12th regeneration at this time, doesn't him going batshit insane and evil-ish worry you now? Reminds you of the Valeyard, doesn't it?
And now we discover that there REALLY IS water on Mars!
Ten's last-second decision to join Adelaide in checking the ice field takes on an uglier tone if you consider that, on some level, he may have been hoping they'd discover evidence that the drinking water was infected. Why? Because then the decision about whether to save everyone would've been taken out of his hands: if they'd all been exposed, then none of them could return to Earth and most likely they'd have agreed with Adelaide's decision to blow up the base with all hands. The "fixed point" would never have been in danger, and the Doctor could've regretfully departed with a promise that A) he'd make sure the Flood endangered no one else, and B) in some future time, the story of what really happened to the Mars base would be told.
This episode shows the Tenth Doctor (briefly) taking the stance that he should be exempt from the Laws of Time and free to change history for the better, because he's the only Time Lord left. This places him in direct opposition to the stance the First Doctor took in "The Time Meddler": the storyline which established that the Doctor wasn't just a lone eccentric who'd invented a time machine, but a member of a (yet-unnamed) race that had practiced Time Travel for many generations and was accountable to those Laws. Which implies it's not just having companions that stops the Doctor from becoming the Time Lord Victorious: it's having his people, however flawed, out there to remind him why he shouldn't.
The TARDIS always takes the doctor where he needs to be, so why take him to Bowie Base One, where he nearly became something he'd always fought against? The Time Lord Victorious, that didn't come from nowhere, The TARDIS knew what the Doctor would do, that he would take these steps if he found himself on Bowie Base One, and it knew that Adelaide would react the way she did, snapping The Doctor out of it. The whole thing was a Batman Gambit on the part of the TARDIS to force the Doctor to confront that side of himself.
