This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Force Unleashed
The Force Unleashed
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)
Kazdan Paratus' death. He was left insane by the trauma, guilt, and isolation he experienced after Order 66, and with his dying breath, he apologises to his dead masters for failing them again.
Shaak Ti's death. After losing her duel with Starkiller, she tries to convince him that he could beso much more. When Starkiller stubbornly insists on his loyalty to Vader, she sadly remarks that he will eventually find out the harsh reality of the Sith before falling backwards to her demise.
Vader's first betrayal of Starkiller.
Starkiller has successfully defeated a Jedi Master, and expects that he will finally be able to do what he's spent his life preparing for: helping his master destroy the Emperor. Instead, Starkiller gets painfully stabbed in the back. His expression of agony, horror, and betrayal is heartbreaking, especially when you remember that Vader is the closest thing to a father that Starkiller knows. In the novel, he's in full-on denial until the Emperor orders Vader to kill him, and Starkiller is completely helpless to stop Vader from following through.
When the Emperor commands Vader to kill his apprentice, Starkiller desperately appeals to his master, insisting that they can defeat Palpatine together. Vader looks from his master to his student, just like he willwhen he finally chooses to redeem himself. Here, however, he savagely beats the helpless Starkiller while Palpatine laughs sadistically and cheers him on, ultimately throwing him out into space to (apparently) die.
In the novel, Palpatine is only present through PROXY, and when the Emperor cuts the transmission, the droid looks stunned and shaken.
Also in the novelisation, Juno is helplessly watching all of this through a hacked video feed. She's left horrified by Vader's cruelty, and is subsequently arrested as an accomplice to Starkiller's "treason".
Meeting Kento's Force Ghost. Especially sad in the novel, where it's made clear that before this meeting, Galen had all but forgotten his biological parents.
Kento: I never wanted this for you. I never wanted any of this for you. I'm sorry, son.
Juno discovering that Starkiller is gathering Rebels under Vader's orders. She looks and sounds so betrayed.
Vader's second betrayal of Starkiller is arguably even crueller than the first:
Everything seems great. The Rebel Alliance is officially formed, with Starkiller as it's leader. Even Kota seems to have found a new life. Then Vader arrives and calls Starkiller "his apprentice", ruining the trust he'd built up in his new allies, before violently attacking him.
Vader's total lack of remorse when he tells Starkiller that he never planned to betray the Emperor, and that he basically sees Starkiller as completely expendable, is a major gut punch, demonstrating that Vader has fallen so far that Starkiller's Undying Loyalty means virtually nothing to him, and he's perfectly willing to kill his apprentice, whom he raised from childhood. This man used to be ahero.
The last thing Starkiller has to say? "Without me, you'll never be free."
PROXY making a Heroic Sacrifice to save Starkiller from Vader. He had been repaired by the second game, but that doesn't make his selflessness in the first any less heartbreaking.
Starkiller's death at the end of the first game.
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.
What makes it even sadder and one of said Starkiller's worst Moral Event Horizons is that he simply casually tosses him aside to join the junk afterwards.
Starkiller actually sounds hesitant in speaking with Proxy, especially when asking for his help. When Proxy helps him leave the room, the man that used to be Galen Marek softly tells him, "thank you" before he leaves.
In the sequel's Dark Side Ending, the last thing Starkiller sees before he dies is Juno's dead body.
A scene in the novelization, where an unconscious Starkiller remembers his mother's Heroic Sacrifice to save her family and as many Wookiees as possible from Trandoshan slavers.
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".
From Bad to Worse: That droid proceeded to brutally beat the clone, who couldn't bring himself to fight back against the machine that resembled Starkiller's oldest friend.
The scene where Starkiller enters the Dark Side Cave and sees a vision of Juno. He tries to embrace her, but she dissolves into smoke. He clutches his chest, his heart breaking.
While making his way through the cloning facility in the second game, Starkiller starts hearing Juno and Kota's voices tormenting him. By the time he manages to shake off the hallucinations, Starkiller's on the verge of a breakdown.
"Juno": Who are you?
Starkiller: Juno, it's me!
"Juno": You're a monster, a thing.
Starkilller: [on the verge of tears] Juno, please!
"Kota": You're Vader's puppet. Just a body filled with the memories of a dead man.