The worst part was that she wasn't even a professional soldier, and was merely a civilian researcher. She also came across as a genuinely nice person that even attempted to talk Kerrigan into sparing her friends and work towards peace while she is held captive. In the end, not only was given a painful and gruesome death, she died knowing that by rescuing her from the zerg, all her friends are now going to die as well. Even Kerrigan was not pleased that she had to do that, if her subsequent discussion with Abathur is any indication.
The scene where Jim Raynor is saved by Kerrigan, only to find out she has turned back into the Queen of Blades, is absolutely heart-breaking, both for the two characters and the players.
Consider the imagine spot at the start from Jim's perspective. We knew what had happened, Jim didn't know Sarah had been reinfested. He hears fighting outside, the door to his cell is blown in, and for an instant he seems to see Sarah, as he knew her, come to the rescue. And then just as he's starting to smile her wings unfold, almost guiltily, while she looks at him sadly.
Special mention goes to The scene where Kerrigan guides Raynor's revolver towards her own head, sorrowfully asking if he still trusts her. Raynor, having just seen the woman he loves back to her zergified looks, is understandably angry at her. having just chewed her out for everything the original Queen of Blades had done, as well as how many people she's killed, including Fenix. We still hear the bullets, one by one, being shot out of the revolver while looking at Raynor's angry face. Then the camera pans and it shows that Raynor has deliberately aimed off course. Despite everything Kerrigan has done, he just can't get himself to kill her—or, perhaps, only stops himself from killing her because if she dies, the universe is doomed. She tries to comfort him, saying that she'll always love him, and two words conveying his anger and disgust are all that he has for her: "We're done". You may cry now.
Not to mention, Kerrigan spends the entirety of the following Leviathan sequence looking away from the player towards the Hyperion, and from 'talking' to the other zerg around her, we hear that she has shut herself off completely psionicly. Click on her, and the only thing you'll hear is her lamentations. She really loved that man, and now, there's nothing left for them.
The whole concept of Amon's corruption to Kerrigan, it was so strong that even after she was deinfested it still controlled part of her, pretty horrible in itself, but the worst is that, to finally be free of it, Kerrigan had to reinfest herself, giving up her own humanity and any chance of a future with Raynor.
Warfield's death, a single mention of Raynor's supposed death, and he gets metal beams jabbed into his body. It's not all tear jerker though, seeing how Kerrigan actually honors his last request, letting his men get out alive.
Yes, one of his men joyfully reports a successful escape...... at the end of the cutscene one can imagine he might have realized that the General won't be joining them.
Kerrigan killing Narud...who just turned himself into her original form. Resulting in a potent reminder that cute redheaded Sarah is now forever gone and Kerrigan's Queen of Blades alter ego has long since become a part of the real her.
The ending: Kerrigan is almost killed by Mengsk when he uses the Xel'naga artifact against her but is saved when Raynor shows up behind him in full marine armor, grabs Mengsk and throws him across the room. Kerrigan then proceeds to blow Mengsk up. Good ending? Yes and no. Sarah thanks Jim for everything and even does it with a smile, to which Jim responds that it was his pleasure, and always was, with a smile on his own as Sarah flies away back to her Leviathan. The kicker? They can never be together no matter how much they love each other because of Kerrigan's role in fighting Amon, aka the Dark Voice and Fallen Xel'naga as well as the one who corrupted the Zerg.
The final sequence in which Kerrigan reflects on all that happened to her and all the she had to give up in the way.
Valerian not being the least bit surprised that his own father is willing to let him die if it means killing Kerrigan is all kinds of depressing. Poor guy must have had a horrible childhood.