I was doing some testing to see if I would be able to create a zombie villager converter, and found that baby zombie villagers will kill the villager rather than converting them to a zombie villager. I turned a single player world onto the Hard difficulty setting, which if I'm not mistaken has a 100% villager to zombie conversion chance, and then killed a villager using a baby zombie. I tested it multiple times and the baby zombie never converted the villager into a zombie villager.