I don't know the particular case, but it sounds like this is just a matter of terminology you are referring to. There's different terms for different types of multiple killings. There is a difference between 'serial killings', which go on, regularly, basically indefinitely. And someone who goes on a 'killing spree' - which is a series of murders in a relatively short space of time, which then stops and is not likely to go on indefinitely. I think maybe there is a third category too, maybe it was 'mass murder' - where a bunch of people are killed in one incident.

As for the teen incident - I haven't heard about it other than here - but while I don't excuse the act at all, I think one needs to also look at the parents' role in it (both the parents of the teen and the baby - assuming the teen isn't the baby's parent that is).