Before the kill attempt:
$ docker-machine ls
NAME ACTIVE DRIVER STATE URL SWARM DOCKER ERRORS
client-hyperkit-vm - hyperkit Stopped Unknown IP address is not set
Killing attempt:
$ docker-machine kill testmachine
Killing "testmachine"...
Machine "testmachine" is already stopped.
$ echo $?
1
$ docker-machine ls
NAME ACTIVE DRIVER STATE URL SWARM DOCKER ERRORS
client-hyperkit-vm - hyperkit Stopped Unknown IP address is not set
The steps on how docker-machine is installed, how to reproduce this issue, and the events that lead up to this issue is detailed within this issue: machine-drivers/docker-machine-driver-hyperkit#15
Creating a ticket because there is another similar issue that is slightly different but almost identical.