The moment Nero agreed with Zatiel's plan back in the forest, they acted immediately. The first thing they did was search Dimitry's mind for all the knowledge he had about Verius and his group. It was with that information they learned the identity of every single member of the Fallen who had sided with them.
Something that shocked Nero was to find out that Verius had made sure in case something happened to him, some of those Rank 3 Fallen would inform the Heavenly race everything they know about the Fallen.
The seven winged Fallen believed he understood the limit of Verius, but only when he discovered that plan did he realize how wrong he was.
That information made his decision of killing Verius and all of those who sided with him much easier.
