What really sells it is Spartacus' reaction. After killing his best friend, he's back in his quarters. He's clearly struggling to hold in his feelings. Finally, he snaps and smashes every breakable object in his room until he's left pounding the wall until his hands are bloody. When Mira finds him, he's weeping like a child, and the episode ends as he cries on her shoulder.
Really, all the main character's deaths count as this.