I don't think that spanking would work with this kid anyway, because she would just dig her heels in and treat you worse. I do think that time outs don't work because she truly doesn't care.
Kids who don't get positive attention seek out negative attention, so it sounds like that is what she is doing. I am not saying that you don't pay attention to your child. I do think that you need to capitalize on what she does well and try to cultivate that with her, and pay attention to that.
What is her "currency"? In other words, what does she enjoy doing, playing with, having? THAT'S what you need to go after when she talks terribly to you. Does she have a room full of toys? When she tells you NO when you tell her to do something, tell her "That's strike one." Don't say anything else except to tell her again to do it - turn on the timer and tell her that when the timer goes off, she needs to do it (give her 5 minutes on the timer, walk away, and give her time to think about what will happen if she doesn't comply.) When the timer goes off and she doesn't budge, tell her "That's strike two." Ditto until she gets strike 3. Then go in her room, without a word, and remove what she likes the best. You need a box that gets put up where she cannot reach and doesn't know the location of that's where you keep all her favorite things. She may not realize for a few days that her stuff is disappearing. When she says "Where is my doll?" tell her that her doll is in time out because she had 3 strikes. If she says she's going to call the police, laugh at her.
Depending on where you live, I would possibly consider going down to the precinct nearby and talk to one of the neighborhood policemen there. See if you can bring her in and let them talk some tough turkey to her about her behavior and how the police expect her to obey her mother and that she can get in big trouble threatening to hurt ANYONE, including her mom. Maybe that would help.
Right now she has you over a barrel because your discipline doesn't work on her. If she ends up losing (over time) every single belonging she has except the mattress on her floor, (including her bedroom door), I think that she will straighten up. I would do this gradually without much discussion and not let her get you sucked into an argument or screaming match. If you just tell her that "x" is in timeout because she got to strike three, and refuse to say anything else, she will finally think TWICE before she says awful stuff to you or says no when you tell her to do something.
You MUST work on this now and not wait, or you will have a teen that end up doing drugs and getting pregnant and flunking out of school. It's this important.
Btw, you are not a bad mom for thinking negative things about your child. And plenty of people successfully manage their children by including spanking in their discipline. Not everyone does, but I don't believe that it would work for you. I do believe that what I am saying here about removing her belongings in descending order of importance to her, and then the big things in her room, is what WILL work. Once you get to bare bones and she has finally started to improve, only then should you start to allow her to EARN back things a LITTLE at a time. (Starting with her LEAST favorite possession.)
Sending you strength~