Yep, you spend your time munching some tasty grass, maybe a mushroom or two. But then a lion suddenly comes and kills Mummy. Well, that can't be good, can it? But when it's all squared away, the lion has to eat too, doesn't it? It can't eat grass like you and can't go to the store and get fifty chickens for tonight's feast.
So it needs to eat you, so sorry.
On the other hand, the below DOESN'T HAVE TO BE DONE.