A crime scene investigator must document a scene objectively for at least two major reasons.
First, investigators must document the scene objectively in case their first impressions of what has happened are incorrect. Investigators may come to feel that they know what has happened when they first view the crime scene. They might then document only those pieces of evidence that are important to their theory of what has happened. The problem can come later if their first theory turns out to be wrong. They then have lost the other evidence that did not seem important at first. They will have no way to devise a new theory or to support that theory using evidence from the crime scene.
Second, a subjective documentation of a crime scene is more likely to be susceptible to picking apart in court. If investigators only focus on the things that support their theory, defense lawyers may be able to introduce doubt. They may be able to allege that there was other evidence that the investigators overlooked that would have proved their client’s innocence. If the scene has been documented objectively, overlooking nothing, this tactic is much less likely to work.
For these reasons, it is important to document a crime scene objectively.