Nature rears its ugly head

I suppose in my heart of hearts I knew not all birds eat worms to survive, but why did a horrible brown carnivore bird decide to kill and eat another in my back garden ... I can see the whole thing out of my patio doors, which I had opened to let fresh air and nature in - never again! This kawk/kestrel/whatever is EATING something in from of my eyes, which are now hiding behind the blinds and occasionally peaking at to see if it is still there.
When I first saw the noise I was going to scare the big bird away, then I thought I might be left with a wriggling dying bird in my garden, so decided to let nature do what nature does.
Gross.
Why can't they shop for their meat in pre-packaged cellophane wrapped blandness at Tesco like the rest of us?
Just looked up google images ... I think it is a sparrow hawk.