> Q: What happened

The passwd lock file, /etc/ptmp, got left behind because vipw had no
chance to clean up.

> how to unlock the /usr/bin/passwd

rm -f /etc/ptmp.

> and what to do in situations like this one?

Try a regular kill of the vipw process before a kill -9?

Side note for developers: when I redid the passwd file locking (the
old locking discipline was broken; it fcntl-locked the master.passwd
file which got removed, so different processes could be locking
against different objects) I originally wanted to fcntl-lock a separte
file (e.g. /etc/master.passwd.lock), but mycroft preferred this scheme
because it would work over NFS.  Does anyone NFS-mount their /etc
directory?