When you have been arrested and charged with murder, it is a common tactic for the district attorney to pull up any past related convictions or wrongdoings in order to present a pattern of behavior. It feels like a personal attack, and it is. However, the "ends" that the district attorney is after is not just to make you look bad. He or she has another motive which they do not have to reveal.
As such, you are left feeling awful and knowing that, with this target on your back, you cannot defend yourself alone. You need a criminal defense lawyer if you are even going to have a chance of proving your innocence. Here is what you can expect in court, and how your attorney will counter it.
Past Crimes of Assault, Disorderly Conduct, Domestic Abuse, Etc.
Any past crime for which you were tried and convicted or tried and acquitted will be brought up in court. If you were really young when these things happen, then your lawyer will argue that you are a reformed person. Crimes as a juvenile are not admissible because juvenile records are typically sealed, giving access to only those who were in the courtroom at the time of the juvenile criminal hearings.
Any crime after the age of eighteen, however, is fair game for the district attorney to use in court. If you have more than one or two criminal cases on your record after age eighteen, and any of them can point to a "murderer in the making," the district attorney will use it against you to establish a history of violent behavior. Hopefully, your court appearances are really old and very few, making them virtually inapplicable to your present case.
Your own lawyer will use the legal tactic of redirection. This means that whatever awful detail the district attorney points out to the jury, your lawyer will redirect the jury's attention away from those details. He or she may point out how good of an adult/person you have become, how you have not done anyone any harm in years (if at all), and how respected you are among your peers. Character witnesses will testify to your good character, and any incidents where you saved someone or helped others in times of need will be part of the redirect process. In short, it is your lawyer's job to refute evidence, paint you in a good light, and make the jury see that you are not a murderer.