Earlier quoted context omitted.
WRT motive, the fact of whether it was or wasn't beef when introduced into evidence may prove to be exculpatory (if the finders of fact decide that it was justified) or may prove to be further damning (if the finders of facts decide that it was pre-meditated). I wouldn't want the police on the scene to be in the business of deciding what possibly relevant facts in the case will prove to be crucial. Collect, catalog,…
It is not illegal and even if it were it does not, in any lawful society, justify extrajudicial torture and/or murder.
It seems no different than taking any other piece of crime scene or crime-related evidence to me.