Whether culturally or genetically, I'm programmed to respond by feeling protective if you put someone young and/or female in danger. Show us that she is in danger and kills to protect herself, and the only problem most people will have is that they will not like the society that puts her in a situation where she has to kill to survive. They won't hold it against her.
You may have to be careful if she feels no remorse and it's her environment that forces these actions on her. That could easily lead to pity, which is probably not what you're looking for.
So if you have a real reason for the main character to be a vile person then the reader can have the sympathy without having to continually justify it. I would imagine that anyone who could kill with ease would have already adjusted to the fact that is the way they are. And to continue to do kill requires that the person to some degree enjoys it. Maybe they like the fact that they now have supremacy over stronger people, and the motive for this is some form of abuse earlier in life that the character was made to feel subdued by a stronger person.
There are many reasons that could drive someone to enjoy killing, or feel it is necessary for survival. Giving a reason for thier attitued towards life can solicit the sympathy you want.
[This message has been edited by Lord Darkstorm (edited August 13, 2003).]
But a motive per se is not enough. It needs to be a motive we can sympathize with. Compare:
In the fourteen years of her life, Jenna had been forced to kill seven men just to survive without being raped.
In the fourteen years of her life, Jenna had been forced to kill seven men just because they didn't prostrate themselves quickly enough as she walked past.
Either way, she's got a motive for the killings. But readers will sympathize with Jenna I, not Jenna II.
Now, of course, it would be possible to get readers to sympathize with Jenna II by explaining the society in such a way that we understand why she must kill men who are slow to show her the proper respect. But that's tougher than giving a motive we can readily understand.
You might think it would be difficult for readers to sympathize with a six-year-old kid who beats up and kills a classmate. But since OSC puts us inside Ender's head so we can understand why he was doing what he did, we can sympathize.
Lets say there is a young girl that was born in the poverty part of a city. And for whatever city, whatever setting, there are people who would pay to spend a few hours with young children. Now you have a motive for a child be seriously messed up. Now if the parents are selling time with thier kid(s) to pay for thier addictions you throw in even further motive. Now lets say the child learns that death is a means to an end, it works. It may be repulsive and wrong, but at this point the child has no sense of wrong since they have been being subjected to wrong all thier lives. So once they find a means of ending thier own suffereing, it now becomes the main form of solving their problems. When in doubt, kill them, problem solved.
Ok, just one example, but it could be made very belivable and it could inspire allot of sympathy in conjunction with some disgust for the main character.
So all murderers that feel they have to kill to survive do not have to be a vampire or something in the same vein.
Eric, now you've made me want to write about a character like Jenna II. A noble, no doubt--perhaps the heir to a throne. By law, all of lower station must prostrate themselves in her presence, and the ruling family believes that failing to enforce the law will generate rebellion and anarchy (perhaps they are right!). Jenna does not feel her royal position is all that important, but perhaps she believes the same as her family. Or perhaps her family will know and punish her severely if she does not uphold the law...
Gah!
[This message has been edited by Maccabeus (edited August 13, 2003).]
By the way, OSC starts one of his books off with a 14 year old female assassin, who kills people (some of them perfectly nice people that she liked just fine alive) on the orders of her government. She is, of course, the main character. Card can be a sicko that way, but he generates sympathy for characters engaged in strange activities all the time (see Card's short story Holy).
Anyway, Darkstorm and Eric are right about the point of showing her as having good motives. She doesn't actually have to have a good motive for killing per se if it isn't a conscious act on her part (I mean, if she can't help herself), but she does have to be presented as a good (and likeable) person in most other ways, including the various suggestions that she have suffer from feelings of guilt over killing people. She should also have attempted to not kill in the past, and make a conscious effort to kill as little as possible.