Doris Lawrence witnessed Christian's death and she continues to carry the guilt she felt towards him. But the only thing she loved in her heart was Snape. Whether she was standing in darkness or light, she didn't even know. During the year of being a spy, how did Snape spend this miserable time. Will Doris stay with him or leave him?
Without Lily, there would be no such flesh-and-blood Snape! So there is Lily in this article, trying not to discredit anyone.