Okay, Starcraft I Kerrigan was perfectly fine as a secondary antagonist character.
Brood War Kerrigan is where the problems started where she was granted a **Mary Sue (TM)** patented Aura of Retardation which made everyone around her drop the idiot ball,
||PS: The UED did nothing wrong||
||PPS: Aldaris was right about Kerrigan and Razagal #JusticeForAldaris||
Then Starcraft II happened, and the Zerg became a race of misunderstood abused puppies, the previous awesome big bad was retconned into being a tormented mind-slave created by their new _super awesome Space Satan Cthulthu OC_ villain and Kerrigan became the prophesied Chosen One.
Fuck Starcraft II's story, it sucks. The whole thing sucks, even WoL doesn't escape it. Hell that's the one that sets up most of the retcons.