Here's another way, I use it in MP for respawn safe zones:
make a trigger, activated by the side you want to kill;
Set it at repeatadly,
and name it (this example uses a name of y1)
then, in the on activation field, put
"_x setdammage 1" forEach list y1
When the player enters the trigger y1, he is entered into the list and killed.
If you want a closed in area, set 4 triggers, name then like y1, y2,y3,y4, then change the on activation field to reflect those names like above. Then set your size of the triggers to box them in. Also used to create a killzone like respawn safezone, using just one trigger.
Hoot out!