I feel like the people who make the comparison has never played Brood war or Starcraft 2 to begin with.
Sylvanas has almost always had free will, Kerrigan was mind controlled by first The overmind, and then amon.
I don't need to list all the horrible things Sylvanas has done throughout the years, but what needs to be said is that she did all of those things on her own, no one was forcing her hand, except maybe Gilneas but she still went full nuclear.
Kerrigan however was forcefully infested, then mind controlled as the queen of blades into murdering billions. Even after the overmind died Amon assumed control over her into killing even more as seen in WoL.
You can't seriously tell me that you cant see a pretty drastic change of character from the time she was the queen of blades to the time she was Primal kerrigan. She expresses regret and apologises for having to infest a protoss scientist or whatever to stop the golden armada. She goes to hell and back to save the love of her life and after wrecking Korhal to kill Mengsk (Rightfully so), she literally takes her swarm to leave without any further bloodshed at all. She could have ended the Terran Dominion right there and then, she didn't. She even gave Raynor the oppertunity to kill her. And even at the end before being turned into a Xel'naga she says "I must do this, there is too much blood on my hands."
Sylvanas however has had free will since the first time saw her in the frozen throne. She goes on murdering sprees all the time, nukes gilneas and lordaeron, literally abandons Nathanos, her love. All of this she did with free will.
How exactly is the comparison fair? Kerrigan was mind controlled and still expresses regret for what she did, and is pretty much an anti hero. Sylvanas has always been evil, even after breaking free. She allied with the literal god of death, Kerrigan never willingly allied Amon, after killing mengsk she went straight to kill Amon. So what gives???
Yes their stories are similar, but so is pretty much everything from the Diablo, Starcraft and Warcraft universe.