Let's try a thought experiment:

Assume (contrary to law) to get sentenced, not only you need to prove that someone did the killing, but also who the victim was.

This could lead to the following scenario:

• Alice kills Bob.
• To make sure nobody recognizes Bob, she chops off his hands and head, then throws both into a snowplow.
• To evade DNA evidence, she takes the resulting mess and the rest of the skeleton and burns them to crisp.
• Nothing to identify the remains as Bob's is left.
• The resulting head- and handless skeleton without DNA she puts up as Halloween decoration.

As it was made impossible to identify Bob with the remains, Alice can't be sentenced under this hypothetical law. In fact, its very setup does not only benefit such gruesome behavior to disguise the identity of the victims, it encourages it.

How the law works

Now, the thought experiment shows how gruesome the results would be if the law demanded identification. But the murder statutes are - in Common Law, Code Civil and Germanic tradition - written in ways that make the identity and even what the killed person did, in general, is made irrelevant, they focus on the mens rea of the one conducting the killing^{aside from self-defense}. Some examples:

• The UK does not define what is murder in a statute, but whathas laws which killing of someone is not murder by pointing to circumstances and the state of mind and what it is instead then.
• NY-State defines variations of murder based on the intent of the killing and the circumstances (Felony Murder Rule)
• Germany has its homicide crimes in StGB §211 to 213, which require the act of killing and an investigation into the intent and circumstanced to decide what section applies.
• France defines its (baseline) murder as "willful killing", so they require the act and the mens rea