He didn't see Sanguinius death, didn't know how the fight panned out and how brutal it actually was. When lone Guardsman/Imperial Fist/Custode runs in and Horus destroys him with a look of enjoyment on his face is when the Emperor realises he truely isnt coming back. From a raders perspective a random running in wouldn't be moving as much as someone like Amon running in and getting so brutally killed. Over the series i've no doubt they will establish Amon a bit more, consumate survivor, likeable, loyal etc, i already like him as do several other readers i know and he's been in virtually none of it. To see a character like that or any established character getting killed in such a way will tug at you a little more than Joe Blogs