Now that you've reached sanctum you are going to finally get to fight Radec, but not before you've exterminated his personal guard. They will storm in through the main doors, so hide behind cover near Rico and shoot them. It's a standard fight so far, but when Radec tell his men to shoot the chandeliers make sure you are not standing under any.
After this stage of the assault the stairwells to the upper mezzanine level will open up so run up there and find two weapons racks - both containing Grenade Launches. Use the launchers to pound the enemy that are on the ground floor and pick up the Sniper Rifle to take care of the Helghast that will assault you from the far end of the mezzanine.
Pick up the Assault Rifle to finish of the rest and get ready to fight Radec. Ever had a boss fight against an enemy that has a personal cloaking device?
Well you will now since that's exactly the technique Radec chooses to use. He will seem to appear out of nowhere and does seem to enjoy trying to get close to you so he can use his knife.
Keep on the lookout for him and walk backwards when possible. Radec will appear and you will be able to get in a few shots before he vanishes again. Keep on the move, don't let him sneak up behind you and you'll not find his demise that tricky.
Now that Vadec is dead move on to find the Emperor who will tell you that he is the only one from stopping the Helghast from somehow being a lot worse than they already are.
Killing him would leave a vacuum of power that would be filled with something far worse, which is excuse enough for Rico to shoot him to death on the spot just as more massive Helghast ships arrive to tell you that there is certainly going to be a Killzone 3. The war is far from over, but this chapter is defiantly at an end.