It sounds to me as if what he did could be considered witness intimidation. Lawyers are considered to be officers of the court, and are held to a higher ethical standard than police officers, who are allowed to use deception to gather evidence.
This prosecutor is probably lucky that the only thing that happened to him was that he got fired. He could just as easily have been referred to the Bar Association for sanctions, including disbarment.
He manipulated witnesses with lies calculated to cause anger, mistrust, and antipathy towards the defendant. His actions were unbecoming of an officer of the court.
You're assuming the defendant's guilt. Instead, consider it just as likely that he didn't commit the murder and that his alibi was legitimate - he really was somewhere else when the murder was committed and his girlfriend is a witness to that fact.
Now make up a false story telling the girlfriend he was cheating on her and even had a baby with another woman. She "goes crazy" and now lies and says he was not with her when the murders were done.
How is this serving justice?
It is not the job of law enforcement to manipulate evidence, it is their job to gather evidence.
This prosecutor is probably lucky that the only thing that happened to him was that he got fired. He could just as easily have been referred to the Bar Association for sanctions, including disbarment.