To kill off slapd safely, you should give a command like this:

kill -INT `cat $(ETCDIR)/slapd.pid`

Killing slapd by a more drastic method may cause it's databases to be corrupted, as it may need to flush various buffers before it exits. Note that slapd writes its pid to a file called slapd.pid in the directory you configured in slapd.conf file, for example: /usr/local/var/slapd.pid

Slapd will also write its arguments to a file called slapd.args in the directory you configured in slapd.conf file, for example /usr/local/var/slapd.args