If they accidentally shot her that's why. So the best option for them was to re-arrange the scene.
So the raid team were told there was a 'nutter' inside. Who knew how to use guns.
They shoot 'nutter' Sheila in the kitchen which is what they are trained to do. Then decide to take her upstairs and make it look like murder/suicide.
Murder/suicide is accepted. But then one month later the police decide to frame Bamber.
Interesting.