Z's are hard too kill, I find to kill them (silently) with an axe you have to have them 45 degrees from yourself, not right in front. I had one the other day that was standing at tent entrance and must have hit them 20 times before they went down, enough time to get my phone out and set to video and record me hitting them another 10 times before they went down...
And they can hurt you while still a distance away from you with their lunges..