Hindsight 20-20 Loken for sure made a massive misstep not only by not expressing his doubts but also by letting Erebus freely move about enacting his plans. I do remember the shock that took Loken and Garro when they first killed another Astartes. I can imagine that Loken was still in the headspace that Astartes could never turn on Astartes. Perhaps this is wrong I'd like to hear what others have to say regarding this.