Objectives: Shut down the disruptor field
The game Dragon Warrior for the original NES had an interesting scenario near the end. After a long journey across many different continents, the Dragon Warrior finally comes face to face with the true form of his arch-enemy, the Dragonlord. The Dragonlord offers the Dragon Warrior a choice: a fight to the death, or dominion over half of the world if he accepts an offer to join forces with the Dragonlord. The catch? If you actually choose to join forces with the Dragonlord, he kills you and the game is over. So, in effect, it's a Hobson's choice: you appear to have free will, but in fact, the semblance of a choice is worse than no choice at all.
Luckily, the folks at BioWare apparently didn't form their design document for KOTOR around the tenets of Dragon Warrior, as you're about to find out.
Step out onto the roof of the Temple to meet with Bastila. No matter what you say during the first part of the conversation, you can't prevent a confrontation, so you may want to activate your energy shields before you get close enough to trigger the confrontation, and have all of your characters using items that can prevent mind-affecting attacks. Your conversation choices will still affect your Force alignment, but not as much as what's about to occur.
You knew this was coming, didn't you?.
At any rate, Bastila will attack you. Her insanity attack has a DC of 31, so it'll be difficult for any of your characters to save against it naturally; this is why having a nerve amplifier belt or something similar on your characters is a good idea, since immunity to mind-affecting attacks will render insanity useless. Bastila isn't very tough in actual combat, luckily, so activate speed and engage her point-blank.
When her vitality drops to around half of her meter, Bastila will stop fighting for the moment and offer you the last critical choice of the game. You will be able to either continue your attempts at returning Bastila to the Jedi Order, or acknowledge your true identity and become (again) the Sith Lord and would-be ruler of the galaxy. And yes, it's an actual choice. If you persist in your attempts to turn Bastila, she'll eventually run off to her ship and escape to the Star Forge.
If, on the other hand, you really want the power to destroy an entire planet in response to the pettiest of slights, then you do indeed have the opportunity to fall from grace here, or fall further from grace, if you've been a really bad boy during the preceding events. The choice is obvious: side with Bastila, and accept her offer to join together as the new Sith rulers of the galaxy. Jolee and Juhani are mildly alarmed by this turn of events, so kill them.
Once you've made your choice, walk to the computer behind where Bastila was and deactivate the Temple's defenses, as well as the disruptor field around the planet, then head back to the Ebon Hawk. If you sided with Bastila, Carth acts like a baby after being told of the little plot twist. The rest of your party members will have differing reactions to your mood swing; Mission, however, is the only one with real objections. If you really, really want to be a bastard, use your Force persuasion to have Zaalbar kill his best friend.
Whichever path you decide to take, you'll need to enter the Hawk, then find the hyperdrive at the rear of the ship and click on it to repair it. Take the ship up to the Star Forge, which should appear on your Galaxy Map.