Ending and killing are the same thing. In Android, if you stop using an app, it sits in memory, doing nothing. If Android needs that memory space for another app, it overwrites the unused one.
Google spent a lot of effort making this stuff work right - let Android do its thing. If you want to go to another app, but get back to the one you're running (like switching between your password app and the browser you need the password in), use the Home key. If you're through using the app, use the back key, or use the Exit (or Quit) choice in the app's menu.