That was the one word that broke their friendship. Lily knew that Severus was angry at James, Sirius, Remus and Peter for what they did and she accepted that Severus didn't mean it. She wanted to forgive him, she really did.
She remembered him coming after her and apologizing his heart out and hiding the tears she knew were stinging the back of his eyes. She knew he deeply was sorry and wanted nothing more than to take back that hateful statement he said and that ugly word at the end. Lily knew with all her might that she could forgive her dear friend and would if not for one thing that was holding her back.
Her pride was spitting and gnawing at her while saying that Severus didn't deserve to be forgiven, that he broke their friendship with the hate filled word of the racial slur that was 'Mudblood'. Her pride was biting her and keeping her back as it told her that he said it in front of all the other students and they knew she broke their friendship. They would call her weak and petty for taking back a young man that called her Dirty Blood in front of them.
She sighed and looked at the shadows of the Slytherin table, seeing her friend and long time crush sitting there, looking dead to the world. She watched him slip away from the Great Hall and away to god knows where that was in the school. She desperately wanted to run after him, grab him and tell him that she forgives him and tell him her deepest love for him, but her pride still bit her and told her no.
No matter what anyone would say in the future, Lily had on thing to say to her pride that would remain for the rest of her days.
'I really hate you.'