The Emperor didn't need to see Sanguinius die to know what happened; they are father and son with a connection deeper than anything anyone merely human has ever felt. For Horus to kill his brother, he would have brought to bear power, focussed his mind and strength and turned to kill a fellow Primarch. The Emperor would have felt all of this, would already have seen in his mind what has happened; he can try to frame this as two of his sons, famously headstrong and stubborn, as fighting as only brothers can- one is fighting for his father, one fighting because he thinks his father is wrong. Horus is only mistaken, though, isn't he? Surely his father can find out what's wrong and ease his mind, say the right words and make it all right.
But, Horus is killing his father. The Emperor can't bring himself to do what needs to be done. He knows his son is sick, but Horus is the brightest and best, the future; the Emperor just has to hold on, the solution is there. And then, through the door through which the Emperor himself walked only minutes ago, a lone Astartes runs in (I'm sure it's an IF who comes in, rather than a Custodes; if it was a Custodes, the Emperor would know his name) and charges in to try to save his Emperor. What threat is he to Horus? None. Against a Primarch an Astartes could do naught; against one infused with the power of the chaos gods, such fury would be a breath in a supernova. Horus doesn't need to pay any attention, the IF is already dead, he just doesn't know it. Horus doesn't even look away from his dying father. He probably doesn't even register the non-threat visually at all. He could just bat the man away, but he doesn't. Instead he turns his power on him, flensing the flesh away layer by layer, pain and cruelty all as one. There is no point to this. There is no gain. And if Horus will do this to one who is no threat to him at all, what will he do to those who are not Astartes, even less of a threat? And so, even though he knows Horus killed his brother Sanguinius and is responsible for the deaths of billions of others, it is the death of this one man that finally breaks the spell that clouds the Emperor's mind. And he does what he always hoped he could avoid.