имхо не девятым сигналом убивать нужно, а хотя бы(для приличия :) -HUP
TERM и INT тоже могут подойти. А если -9 - то конечно и pid-файлы останутся и радиус ничего не узнает.
По крайней мере в man pppd написано:
SIGINT, SIGTERM
These signals cause pppd to terminate the link (by closing LCP), restore the serial device settings, and
exit.
SIGHUP This signal causes pppd to terminate the link, restore the serial device settings, and close the serial
device. If the persist or demand option has been specified, pppd will try to reopen the serial device and
start another connection (after the holdoff period). Otherwise pppd will exit. If this signal is
received during the holdoff period, it causes pppd to end the holdoff period immediately.