The death of Kento Marek, as he fails to protect his adopted world and his son. Made worse that Galen's assistance only bought him a few seconds.
Darth Vader: (Force Chokes Kento) I sense the presence of someone far more powerful than you nearby. Where is your Master? Kento: The Dark Side has clouded your mind. You killed my Master years ago. Darth Vader: Then, you will share his fate. (raises his lightsaber, only for Galen to steal it with The Force) A son? Kento: Run!!! (Vader closes his fist, snapping Kento's neck)
Meeting Kento's Force Ghost.
Kento: I never wanted this for you. I never wanted any of this for you. I'm sorry, son.
The first time Vader betrays Starkiller, then the second time he betrays Starkiller.
In The Ultimate Sith Edition, finding PROXY's broken remains on Tatooine. Also, the death of Obi Wan Kenobi and Luke Skywalker's fall to the Dark Side.
In the sequel's Dark Side Ending, the last thing Starkiller sees before he dies is Juno's dead body.
Behind-the-scenes notes give the fate of one of the failed Starkiller clones:
Lead Cloning Technician: Curious. One of the more stable test subjects began weapons training today. A training droid shorted out and wouldn't project the training program. He was having no problem cutting down all the other droids, who appeared to be living human beings, but he refused to attack the damaged droid. Flat out refused. Instead he smiled at the droid, and then started mumbling over and over: "proxy, proxy, proxy".
The scene where Starkiller enters the Dark Side Cave and sees a vision of Juno. He tries to hug her, but she dissolves into smoke. He clutches his chest, his heart breaking.