Why it would make sense: There’s no better way to prove you’re an evil son of a bitch than by butchering a pregnant woman with a baseball bat wrapped in barbed wire. So Negan could score several evil points there.
Can you imagine? That would scare the living hell out of every single person there. Even those whistling choir of Saviors.
Also, and I’m no prenatal expert, but it seemed like Maggie was in a fair amount of danger to begin with. Perhaps Negan put it best, himself, when he said: Jesus you look shitty. Let’s just put you out of your misery right now.
Why it wouldn’t make sense: By killing Maggie, you’d also be killing Glenn. Figuratively at least. And Negan strikes me as the kinda guy who wouldn’t want to dilute the strength of the survivors he has left.
Final take: Maggie lives. Her baby, however? Seems like a long shot, no? But if that baby does somehow survive all this, he’d go on to be the biggest stud ever. He’d make Carl Grimes look like Freddie Benson from iCarly.