When Maximus is first taken out to be killed up in Germany somewhere at the beginning of Gladiator and they ride out until dawn and are to slay him, he goes all amazing and escapes.
Now at this point he decides it is in his best interest to race home and try to beat whatever envoy/runners the emperor has sent to kill his family (and doesn't make it.)
Now I know if he did this there wouldn't have been a movie, but why not ride back into the main camp (even if it was mobilizing it's not like a camp of that size could disappear in a day) wrangle up your loyal army, rout the small contingent of soldiers loyal to the emperor, and slay him, then send your best and and fastest riders to catch up to the runners and inform them NOT to relay the death order for your family.