Why do we have to let people die a natural death if they are obviously suffering?

my mother who is 85 got cancer last year she was told chemotherapy was the way to go. she has been going since march - and is now 5 stone, blocked up totally one end and still being fed and watered. we don't do this to animals. I think the law sucks and its makers are old fools. I think doctors are all hypocrites and sadists. I hope all politicians go to their hell - very slowly and painfully. and yes I am angry.