I chose to spare him because I didn't want to give him what he wanted.
Before the decision to kill Randall, I wanted to kill him so badly. He annoyed, he was a coward, hit women, tortured kids etc. But then as he went on about his rant to Michonne, I realised that he wasn't worth it. He tried to use Michonnes daughter as a way to suck her in to kill him. He already thinks Michonne is cold and is a cold blooded killer. Yes, he can be used as a bargaining chip to Norma, but imo, being the bigger person and walking away from Randalls sly tactics to kill him was the smarter move to me. I mean Danny and Andy St john were baiting Lee to kill them and given what they had done, they deserved to be killed but giving them what the want would be easy and letting them suffer was the better choice. Plus, a lot of people I feel didn't kill them because Clementine was there. I feel had Michonnes kids been with her, most of those people who killed Randall, wouldn't of done it.