I really want to kill her. She hates my current characters race.

Finish the Sanguine deadric shrine quest and the Thieves Guild "Adarji's Heirloom" quest, then she becomes un-essential.

Yes, there is such a command.

~ SetEssential 34E8C 0

34E8C is the BaseID for Countess Alessia Caro
0 is what makes her non essential (1 would make her essential)

There are a couple things to bear in mind. She is required for a Thieves Guild quest and a Daedric Shrine quest. Both are detailed in the second link below. Note that, as schmoe points out above, there is a point in the game after which she becomes non essential anyway.


Here are some reference links that may help:
http://www.uesp.net/wiki/Oblivion:Console_Command_Tutorial#SetEssential_BaseID_.23
http://www.uesp.net/wiki/Oblivion:Countess_Alessia_Caro

For some reason it didn't work. She still is essential.

If you have the PC verison and a Construction set do this: Go to Actors-Imperial-Female she should be the first few to show up at the top, then click on hers (Not Ottus, although she's isn't exactly non friendly to Argonians too, but that one isn't essential) and look where it says Essential, uncheck that and she should able to be killed now.

If not try unchecking Quest item.