Once upon a time, before the world went to hell in a handbasket, Negan lived a good life with his wife Lucille. They both worked together at the high school, him coaching baseball and basketball and her teaching English composition and literature. It was a good life, but more than anything Lucille wanted to have a child with Negan. However, he did not. He made his excuses, claiming there wasn't enough money, saying that he didn't want to share her with anything or anyone. Lucille begged and pleaded, full-heartedly believing that he would be a good father since he did so well with his athletes. But he always refused, and eventually Lucille didn't push the matter any further. Still, Negan went elsewhere to seek his pleasure.
But adultery isn't a crime of passion to commit lightly. In his heart, Negan still loved Lucille, but he did not want to give her a child. He had too many insecurities and fears of whether or not he'd be a good father to his child. He was already a shitty husband to Lucille, too possessive and too selfish. But she loved him anyway, and he loved her. So for her, he would do it.
Eventually, Negan came to terms with it himself and finally gave in. Now he wanted a child with her. He shunned his mistresses and partners, and returned to Lucille, but it was too late. In the span of five years where Negan had been out cheating, Lucille had developed ovarian cancer unknown to either of them. There would be no hope for children, but more than that, there would be no hope for Lucille either. The treatment was too much for her, and she died – but she died with an 'I love you' on her lips just for Negan.
With Lucille's death, Negan's world came crashing down around him. Lucille came back, her eyes glassy and white, her voice a guttural groan as she reached out for him. She was dead – one of those rotting corpses that Negan had heard of on the news – and being too much of a coward, he left. Eventually, Negan found his way in this new world, cutting his own path of survival, but he never forgot about Lucille. He carried her with him in his heart and in his hand in the form of a wooden baseball bat wrapped in barbed wire. His heart became hard and he became a monster, because only the terrible survived anymore. As the years passed, he fell into despair and lost all hope...for who could ever learn to love a beast?