This account was religiously viewed, and understood, by the writers in hindsight. The writers believing God was responsible for every battle won or lost and all plagues etc.
In their view, what God allowed, he permitted, and what he permitted, he granted license to, and what he granted license to, he set his word toward it.
Meaning that God was the de facto port of call in their journey making him responsible for the slaughter. This is a common understanding throughout the Bible. Nothing new about it.
The Faithful and Discreet Slave has failed to properly teach the domestics how to understand and read the Bible.