add "stop" after the second line (send "&cYou can't message yourself!" to executor)
if arg-1 is executor:
send "&cYou can't message yourself!" to executor
stop
should indent it, as well as cancel the chat event so it acts more like a command. Here's something i made quickly.
on chat:
set {_msg::*} to message split at ""
if {msg::1} is "!":
cancel event
set {_msg::*} to message split at " "
if {_msg::1} is "!hello":
send "Hello There!"
else if {_msg::1} is "!rules":
send "1. No Greifing%nL%2. No Swearing%nL%3. No Hacking"
else if {_msg::1} is "!help":
send "Help Page:%nL%!hello - Say Hello!%nL%!rules - Lists Server Rules"
else:
send "Unknown Command! Type !help for a list of commands."