No, not a 'killer app' in the sense of something bigger and better, but (quite literally) a 'killer'. In this case a tiny widget that kills the running instance of Whatsapp on your Symbian smartphone. Why would you want this? Well, Whatsapp does tend to take over and essentially runs all the time, whether you want it to or not. This kills it stone dead - until the next time you need the chat service, when Whatsapp is restarted from scratch and you're back in action.

From the Nokia Store description:

A simple widget to start and stop completely WhatsApp.

Note: 
Content can only be accessed through Home screen > Widget catalogue. No icon in Menu & pressing launch in store client will not do anything! You need to go to homescreen and select add widget.

You need WhatsApp to be installed on your phone.

Here's the somewhat minimal 'interface'(!):

(images source)

Nokia Innovation comments:

The thing I love about it, is that it doesn’t open up another UI screen, it just turns green or red. Why would you want to switch WhatsApp off anyway? Well it can be a bit of a battery drain especially if you’re not using it and don’t need it running in the background. Having complete control over it is exactly what us Symbian users like!

Interestingly, the widget is based on unsigned code from @Huelif and other code snippets are planned to be published soon, including a similar widget to kill all background map services (e.g. to free up RAM or compass functions).

You can buy this widget for a nominal sum in the Nokia Store.