5yr Old Daughter aggressive toward mom

My 5 year old daughter displays negative and aggressive behaviour toward me when she cannot get her own way.  I try and respond in a calm manner, use time out.  She displays aggressive outbursts and says things like "i'm going to kill you mommy".  She is one of 4 children, the second youngest.  I spend a lot of time with her, give her a lot of attention.  I feel that the birth of my son has had an impact on her, now that she is not the "baby" anymore, although, she has a good bond with her younger brother (he is 2yrs).  She doesn't display any negative behaviour toward him, her sisters, grandparents, stepdad or father.  Please help, I love her so much and it is so heartbreaking to see her behave like this.