On Gentoo, apcupsd gets installed into /usr/sbin. If /usr is dismounted during shutdown, the killpower command cannot work. When installing apcupsd for Red Hat, for instance, the executables are installed into /sbin, which is probably still mounted when killpower is called.
A snippet from http://www.apcupsd.com/3.10.x-manual/Building_Installing_apcupsd.html --sbindir=<path> This defines the directory for the executable files such as apcupsd. The default is /sbin. You may be tempted to place the executable files in /usr/sbin or /usr/local/sbin. Please use caution here as these directories may be unmounted during a shutdown and thus may prevent the halt script from calling apcupsd to turn off the UPS power. Though your data will be protected, in this case, your system will probably not be automatically rebooted when the power returns.
It appears that bug 65211 was inappropriately set to fixed. It brought up this issue, plus one I didn't think about with regards to /var/log.
*** This bug has been marked as a duplicate of 65211 ***
