You clearly haven't looked up the definition of serial killer/serial murderer, there is nothing about "enjoyment or fascination" as a requirement for being considered a serial killer in the dictionary. "Serial Murder: The unlawful killing of two or more victims by the same offender(s), in separate events." Usually at least 3 events. According to the FBI definition this event is considered a mass murder since 4 people were killed since a mass murder is if 4+ people were killed. If we count the mass poisoning she took part in committing. If we include her attempt murder, I mean "saving" of Leniel that is 3 separate events we have seen in story. She is a serial killer
According to the very generalized and simplified definition they have to
start considering someone to be a serial killer, you might "technically" be right but if you read
https://www.fbi.gov/stats-services/publications/serial-murder it's not quite so clear cut and there's a lot more that goes into the process of definite labeling someone a serial killer and I'm 99% sure she wouldn't get labeled as a serial killer, mass murderer sure, but not serial killer (not based on what's been shown so far atleast).
The simplified definition would label a gang member a serial killer for having killed two-three rival gang members at separate times, the FBI/cops however would not.
Simplified general definitions are just that, simplified and generalized, which is why the other things they take into consideration like motivivation etc isn't included in the initial assessment.
EDIT: I've read enough definition's from different sources/countries and seen enough documentaries about serial killers to be pretty confident in my assertion that it's highly unlikely that she would be considered a serial killer, even though I'm not an expert.