This plugin very simply allows people without the minecraft.command.kill permission to use the old functionality of /kill.
If somebody without minecraft.command.kill does /kill, they will commit suicide, but they will not be able to kill other players/entities. When somebody with minecraft.command.kill uses /kill, this plugin does nothing, i.e. everything will work as if the plugin wasn't installed.
The reason for using this plugin is because the functionality of /kill has been changed to allow admins to kill other players or entities, but this has the downside of not allowing non-admins to easily commit suicide.
Installation is simple, just drop the .jar in your plugins folder. There is no configuration.