Alternate Ending (SPOILERS)
If anyone was wondering what the alternate ending was when you play as Anakin in the fight on Mustafar here it is:
When Obi-Wan jumps off the thing in the lava river he lands on the riverbank as usual. But then Anakin jumps over him and stabs Obi-Wan through the back and out the front from behind. Anakin then kicks him down the bank and walks away. At the top of the facility he is greeted by Emperor Palpatine who congradulates him on his victory and tells Anakin that there are no Jedi left to stop them and the galaxy is theirs. He also hands Anakin a new red lightsaber. Anakin takes the lightsaber, ignites it, and stabs Palpatine through the stomach and he collapses to the ground dead. The clone troopers are shocked and briefly hold their guns on Anakin. Anakin then says, "No, the galaxy belongs to me!".