She had no choice. Self defense is an "affirmative defense." The burden is on the defense to produce evidence of self defense and the prosecution then must rebut that evidence to show that it was not. She had no choice since there were no other witnesses and only her testimony can lay the foundation for her battered woman defense as well. If she didn't claim self defense she was looking at LWOP at a minimum. This is her only way to avoid 1st degree murder. Her defense team had to have had many discussions with her regarding the risks inherent in her testifying. But at the end of the day, unless her testimony causes the jury to want to give her the DP, she had nothing to lose and a lot to possibly gain by claiming SD and taking the stand. She also probably thought that she could put on her victim face and con the jurors into buying her nice girl persona.