His intellect ranges from below average to above average. He is from all walks of life, as are his victims. The majority of the time, he will kill within his own ethnic group. Or, he may be socially inept, unable to maintain relationships, and a complete loner. The difference is a temporal one.
Serial killers kill several people over a period of days, weeks, months, and even years.
It is not a one-time event. The serial killer kills in cycles, shifting between an active period and a cooling off period. A mass murderer, conversely, kills several people in a matter of hours at most, murdering in an outburst without a cooling off period. Usually mass murderers take all of their victims from one location. Actually, this is the exception with serial killers and the reason why so many serial killers are successful in their endeavor to get away with multiple murders.