Alright, so, here's what ended up happening...
hit roll: 19
Zombie's dodge roll: 4

Damage roll (for rapier): 6
6 + (10/4 + 10/4) = 12 (I prefer to round up, so it's 6 + 3 [usually 2.5] + 3 [usually 2.5])
- 6 from defense
= 6 out of 4 HP in damage. That zombie's down.

I would have rolled for Backstabs, and the zombie who was going to attack me next, but, well, the zombie died before that.