Allowed But Regulated
But thinking about the issue, I realize that in most countries homicide (as in, the killing of another human) is in fact allowed but strictly regulated.
In general, homicide is only legal in self-defense (some backward jurisdictions also allow homicide for "honor" reasons), even if the circumstances where lethal force can be used vary substantially.
Also, most jurisdictions differentiate between cases of deliberate and accidental homicide, and allow for the killing of fetuses - which some people regard as homicide. I am not aware of any place where homicide is absolutely prohibited.
After all, "We should also legalize homicide, then?" isn't much of a counterpoint.
* Murder is unlawful killing, so if it's legal it cannot be murder.