As unlikely that it is for me to ever be in a place that active shooters seem to find so attractive it is necessary to think it through.
If with family, they are number one and we put distance between us and the shooter as fast as possible. If pinned down defend them with everything I got.
If by myself my first instinct would be to stick around and help. However, the heroic attempt carries a huge set of probable negatives:
dealing with cops doing their job (absolutely no disrespect intended),
jail time - loss of income / job,
possible criminal charges,
time in court - loss of income / job,
shooters lawsuits / family,
collateral victims lawsuits / families,
property damage lawsuits,
Remember, it's 2013 USA
.44 Special: For those who get it, no explanation is necessary. For those who don't, no explanation is possible.