I am confused about why a defendant would choose to not testify. A criminal defendant has a right not to testify, and jurors will be told that they cannot assume anything negative if the defendant decides to keep quiet. Of course, some jurors do make assumptions-and they cast their votes accordingly. Evidence of a previous crime may cause some jurors to think that the defendant is guilty of the current crime, too. If the defendant testifies, the prosecutor may be able to bring out other information that tarnishes the defendant's reputation and discredits his testimony.
Some defendants have a poor demeanor when speaking in public.
A judge or jury may not believe a defendant who, though telling the truth, is a nervous witness and makes a bad impression. The defendant may have a perfectly good story which would nevertheless sound fishy to the average jury in that particular locale. The defendant admits that she did in fact commit the crime, but claims that it was justified by the other person's threatening actions. Self-defense is rooted in the belief that people should be allowed to protect themselves from physical harm. This means that a person does not have to wait until she is actually struck to act in self-defense.