LOOK AT THIS ******* GUARDSMAN.
He's spent months fighting a grueling war in which his enemies are demigods allied with daemons, and now he's found himself in the closest thing to Hell he's ever known. He probably wasn't even supposed to get teleported up to the arch-traitor's battle barge in the first place, and just ended up in the wrong place at the worst possible time.
Somehow he's survived horrors beyond comprehension to make his way to the very bridge of Horus' flagship. He saw a veritable angel call upon Horus to answer for his crimes, and he saw that angel die as messily as any guardsman. His Emperor - who he fervently believes is a god incarnate, even if he's not supposed to - lies mortally wounded, and Horus, perhaps, has taken a moment to gloat before he strikes the killing blow.
His armor is slightly more effective than tissue paper, his weapon is slightly more powerful than a flashlight, and Horus' power claw is bigger than his entire body. He stands before a being infused by the dark gods' with incalculable power, that can and will obliterate his soul with no more effort than it would take him to swat a gnat. Nothing he can do could possibly make a difference.
He could run. He could turn his weapon on himself. He could give in to the insidious whispers that echo from the ship's corridors into his mind.
Ollanius Pius does the duty his Emperor requires of him. He dies standing and holds the ******* line.