Q: Kill Background in multitasking ?

Because there isn't.

The way ios does multitasking there is no reason to do so.

There are very few apps that actually run in the background, such as audio streaming and navigation apps. A few will complete a task started in the forground, then stop. The rest are frozen for rapid task switching and consume no resources. The iOS manages memory use without any need to manually "close" them.

iOS: Understanding multitasking

'Im just saying that it would save lots of time.

How can you "save time" more that not having to do anything or think about managing the apps??

The iOS is not like the lame Android OS