1) The apps in the list aren't running - they're the ones that have recently been running (and may or not still be running) - which makes it easier to switch back and forth.
2. NEVER use a task killer in Android unless you've at least gone through the source code of Android. Windows likes as much free RAM as possible, Android likes as little free RAM as possible. You kill a task (not by removing it from the list, that just removes it from the list - it's still in RAM) that the Android memory manager thinks should stay in RAM and Android reloads it. Your task killer kills it. Android reloads it. Etc. That slows th phone down and kills the battery.
3. Try a different launcher. If that works, it's a bug in Next.