Blood traitor.
Really, what she had meant was that you betrayed her blood. The blood that once flew so freely from her to you, nourishing every tiny part of you until you pushed yourself into this world, kicking and screaming to announce to the world you had arrived: Sirius Black III.
Blood traitor.
When you were three, you nearly picked up one of the Dark artefacts your father brought home, and without even thinking she rushed forward and pushed you out of the way, only to receive a severe gash from what looked like an innocent pen. The words 'mudblood' formed on her torso and did not stop bleeding for a week. You never stopped to wonder why mummy was suddenly too tired to see you, you just went on playing. And that hurt much worse than any dark curse thrown her way.
Blood traitor.
After that incident, she fought with Orion for the first time, demanding he shield you from such dark objects lest you get injured. Orion thought she was fussing about nothing, but she was adamant. It was the first time Orion listened to her, the first time she felt power surge through her as she threatened to use the darkest curses she could think of on him if he did not comply. Blood was pounding in her head, as she thought of how she nearly lost you. (Now, she regretted it, maybe if she had not done that you would have turned out differently.)
Blood traitor.
She bought you everything you wanted, gave you books that were not meant for your age because only she realised just how clever you were, she gave you your first broomstick and watched you fly even as her heart did the same manoeuvres you tried out on the broomstick. And yet. And yet, you only glowed when Uncle Alphard showed up (uninvited as always), you only gave that special grin to him. So yes, she always treated him like scum, yes she scowled every time he came, because how was it that your little face would only brighten at seeing his, and all she could ever feel was jealousy pumping through her body. How was it that she was your mother and yet she was never at the receiving end of that beatific smile? She hated her own brother because her son loved him better. (When the time came to remove him from the family tree, she did it with great relish.)
Blood traitor.
When Regulus came, she could finally breathe. Finally, she got her little boy who gazed adoringly at her, her little boy who listened to her, her little boy who did exactly as he was told. Finally. But of course there was still you, and even though she thought she did not want to, she could not stop worrying about you too. Even though you were far from her perfect son. When you were sorted into Gryffindor she nearly stopped breathing out of shame. And still, still her heart beat with worry of how you would be treated, how you would cope with the mistreatment of being a Black in Gryffindor when your stupid morals (oh dear Merlin, who on Earth did you ever inherit that load of bollocks from?) would not allow you to fight back the way all Blacks did - dirty, damaging and lasting.
Blood traitor.
And in the end, when you walked away, with nothing but a look of disgust on your face, when you left to what you thought would be your freedom, she just wanted to rip apart every artery and vein inside her body. How could you make your own mother feel like she was mere dirt? She wanted to scream and scream and scream because how dare her traitorous heart feel like weeping at the loss of her first son, when really, it should be rejoicing of being cleansed and kept away from the likes of him? She grabbed her wand and was about to snip away at every artery inside of her, until Orion quickly grabbed her hand and looked pointedly at Regulus. Regulus who looked as if he himself had died. For him, she stopped, even though you had crucioed whatever was left of her heart by the simple act of leaving. For a witch so powerful to be defeated by the sight of her son leaving her, never had she felt such sorrow and shame at the same time. So instead, she blast you off the family portrait, feeling a strange sort of coldness spread through her fingertips to the rest of her body as she did so. In theory, her heart continued to beat on, her blood continued to flow through her body, but really it had all turned cold. All she could feel now was cold.
Blood traitor.
You were only every right about one thing about her. She did keep herself alive out of pure spite. Hoping that you would finally see that you were wrong, that you had chosen the wrong side. That you were finally losing. And when you did, oh how you would come back grovelling to her! How you would beg her to take you back, to shield you from this world like she once did. How you would finally realise she was all you had. She hadn't decided whether she would or would not take you back at that point, but liked to pass the time imagining cruel ways to make you pay for all the damage you did. What she didn't realise was that she just wanted to see you once more, she just wanted her little first-born to be near her as she aged, as everyone around her slowly passed away (first Regulus, then Alphard, then Orion and Cygnus and oh Agrippa! She was alone, alone, alone with only a house-elf to entertain her), to at least have someone in her family with her.
Blood traitor.
So she waited, and waited, and waited, until her blood finally gave up on the idea of you returning.
Blood traitor.
Truer words had never left her lips.