iOS apps aren't written that poorly. So technically, unless you are jailbreaking, you don't need an "app killer".
Plus to stop the "flame war", iOS has very limited background app execution thread capabilities. IE not a lot of apps can run in background to begin with.
And if you think killing the app out of memory is a good thing because it frees up memory for other apps, you have read way too much bogus hype from the Android world. Since that issue was squashed when 2.1 was released.