As Angel of Blood said though, the Emperor did not witness the death. It could stand as a proof to just how much he did not want to believe that Horus was gone, that he would have to kill him. But seeing the last victim so brutally killed, his flesh and bones and muscles flayed layer by layer, all the while with Horus smiling as he did so and all right in front of the Emperor. Thats a front row seat to witnessing your most loved and favoured son showing you he's gone and something worse stands before you.
Merely seeing the corpse of Sanguinius should have been enough for the Emperor to realize this, it should not have taken another death right in front of his eye(s). But then, the Emperor has been willingly blind and stupid to quite a few things when he should not have been.
Merely seeing the corpse of Sanguinius should have been enough for the Emperor to realize this, it should not have taken another death right in front of his eye(s). But then, the Emperor has been willingly blind and stupid to quite a few things when he should not have been.