Fabius was ruined by his books, especially when you remember that due to the warp time-distortion, less than fifty years prior he was devolving marines into living weapons that he also called “New Men,” was responsible for the EC fall was much as the Blade, Fulgrim and Eidolon were and turned his brothers into the sense freak perverts he supposedly hates. He was the one who pioneered rewiring their pleasure centres.
Even in the audio drama “Repairer of Ruin,” he is talking to a WE, he Igored and removed the nails from.
“War of the Spider” was a much better showing for Fabius, manipulating everyone, making monsters and getting away with everything, including Custodes raw materials.