For automated start and stop:
Edit /etc/init.d/zabbix_agentd_ctl:
#!/bin/sh
#
# chkconfig: 345 95 95
# description: Zabbix Agentd
...
next, configure starting and stopping:
chkconfig zabbix_agentd_ctl --add
chkconfig --level 345 zabbix_agentd_ctl on
Enjoy