On any stock ROM, you don't get the option to kill system apps. You can only kill what you have already ran. The only way you can kill a system app is if you have some kind of specialised task killer app that picks up everything that is running. You'll just mess up your phone if you kill system apps. Of course, it gets technical, because gmail may be a system app but you can kill it safely. The system apps I'm talking about are the ones that run the OS and killing those could be detrimental.