It is still unclear how many apps were killed or what were Google’s criteria for the appocide. It seems that most of the apps were, for lack of a better word, utter crap. Since these horrible apps also tend to have a number of security issues, the move might also help Google fight the spread of malware and spam. It is a bit like draining a pond to get rid of mosquitoes in a malaria ridden village.
Unlike Apple, Google doesn’t vet apps before they end up published at the Play Store. This means that all sorts of terrible apps were, and still are, carried by the Play Store.
Sadly though, we feel Google could delete tens of thousands of apps more without anyone noticing.