After trying multiple times, I have discovered that when leaving a villager and a zombie alone, the zombie will just straight up kill the villager instead of turning it into a zombie villager.

What you thought should happen:

The zombie should just turn the villager instead of killing it completely.

What actually happened was...:
The zombie was completely killing the villagers

Steps to Reproduce:
1. I put a villager in a hole
2. Threw a zombie in the hole 
3. Got out of range of the zombie, he attacks the villager

4. Instead of turning the villager, the zombie kills it immediately.