Mandatory life sentance for murder. Should we keep this or not?

I am writing an essay on the above topic and would like some feedback as to the general view of the policy. As it stands, a criminal convicted of murder, has to receive at least 15 years in prison, it is mandatory. But my issue is , some murderers are being let off as juries are unwilling to p a life sentence(reducing it to manslaughter on a partial defence) and secondly, killings such as mercy killing are being sentenced the same as a sadistic torturer, surely this is not right? Are homicide laws completely out of date? Thoughts please?