What percentage of murders occur between strangers?

Very few. Most crimes are committed by someone the victim knows personally, not by a random person. The U.S. Bureau of Justice has some statistics here: Bureau of Justice Statistics. read more

2010, strangers committed about 38% of nonfatal violent crimes, including rape/sexual assault, robbery, aggravated assault, and simple assault. In 2005-10, about 10% of violent victimizations committed by strangers involved a firearm, compared to 5% committed by offenders known to the victim. read more

"Non-stranger" is a classification of a crime victim's relationship to the offender. An offender who is either related to, well known to, or casually acquainted with the victim is a nonstranger. For crimes with more than one offender, if any of the offenders are nonstrangers, then the group of offenders as a whole is classified as nonstranger. read more

List of countries by intentional homicide rate IF my Math is correct, dern you Gary!, that makes it 12,437 murders in 2013. However, that doesn’t answer your Question. According to the FBI (per Scientific American) of that 3.9 per 100,000, no more than 1% were done by Serial Killers. read more