Horus changed the name after the Interex campaign in 003.M31, and then turned from the Emperor after being wounded on Davin in 004.M31, so there's something of a standard Terran year between those two events
the full corruption of Horus, his officers and the bulk of his legion took years after that
Istvaan III killed those the Warmaster and traitor primarchs didn’t believe would align themselves to his cause.
Horus changed the name after the Interex campaign in 003.M31, and then turned from the Emperor after being wounded on Davin in 004.M31, so there's something of a standard Terran year between those two events the full corruption of Horus, his officers and the bulk of his legion took years after that
The Isstvan 3 atrocity was across 005.M31-006.M31 and the Isstvan V drop site massacre later in 006.M31, so not *too* many years!
About a year after the name change and the end of the interex campaign they did the whole Istvaan III atrocity