Well in my experience serial killers only kill one person at a time over a period of time with cooldowns in between. Thorkell seems like a mass murderer who'll keep doing it til he dies. Also for as monstruous as they're portrayed to be in the news or movies and tv, most serial killers are cowards who dispatch their victims once they've become immobilized and can't fight back. Thorkell is unreasonably powerful for a human, but he prefers that his enemy fight for their lives.