Before the zombie outbreak, Negan was a high school coach. He loved his wife very much, despite having an affair with another woman. When his wife got cancer, he broke off his affair, much to the chagrin of his wife, who asked him why he had choosen the sick one. Apparently, she cared so much for him, she's rather see him happy with the other woman than being sad at her side. Staying at her side in the hospital, the zombie virus broke out. The hospital nurses demanded he'd leave, because it wasn't save there anymore, but Negan refused to leave his wife.
Negan, with other members of The Saviors, ambushed Rick and his small group after they agreed to set camp for the night while they were on their way to The Hilltop to enlist Jesus to help them against The Saviors. After lining everyone up and explaining to them how things were going to be from now on he decided that they should be punished for killing his men and not following his orders. Eventually, Glenn was the unfortunate one to get the short end of the stick.
Negan has a odd but unique personality. He is the leader of a group called the Saviors, he is feared and respected as a king, but his characteristics cannot be dubbed exactly evil. He is rarely seen without his beloved barbed wire-covered baseball he lovingly calls Lucille.
While he can of course be unmerciful, for instance he killed Glen by smashing his brains in with his baseball bat just to simply get a point across. But at the same time he spared both Carl for killing his men, and Rick as well, for attacking him in front of his people. He seems quite reasonable when someone decides to negotiate with him; he was very friendly to Carl and had even apologized to him for making him uncomfortable about his condition. Let's not forget about his use of a hot iron on the face to mark those who betray him.
But despite the fact that he isn't exactly a 'tyrant' and can be quite friendly at times, he still has a habit of provocation and likes to play in power games. He took imminence joy in making Rick look like his "b*tch" in front of his people, and had even teased Rick about his son, which was the cause of the attack.