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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.