I killed one yum process as it was doing an install, and my punishment was quick and severe: yum from that point forward just hang on most commands, doing nothing.  After consulting a variety of internet sites and forums, etc, trying out their solutions, here's what I did that worked (thanks http://www.linux-archive.org/centos/33529-yum-hangs-when-reading-local-rpmdb.html):

cd /var/lib/rpm

rm *__*   [3 db files]

rpm --rebuilddb

Testing:

yum list available