By the additive property of killing, you’d be a triple killer if you killed a double. Serial killers get the title by killing people all in a row. If they’re not in a row it doesn’t count.
The trick to the trolley problem is to yank the brake, jackknife the trolley cars, and hit everyone. This would make you a septuple killer, and earn the ‘Badass Moves’ achievement.
Ok, but what if I drive a Time Trolley to run over my own grandpa before he met my own grandma and kill Antoni Gaudi?