One extension of the sanctity of life belief that is often overlooked and forgotten is that it also is against artificially continuing a life. Basically, it's a "die when your time comes, not before or after" stance. An important distinction is required with life support. In some circumstances, someone may naturally die if left untreated, but with modern medicine, they can be kept alive and a recovery can take place. This is not artificially supporting life. This is using the knowledge of modern medicine the way it should be used - to promote health. However, in some cases, you will have patients that are so far gone that they have no chance of recovery and their body is little more than a shell. Whatever animation is present is created solely by a machine and there is no chance this will change. This, to be, is artificially supporting life beyond "its time". It also seems to be a massive waste of time and money, especially for health services suffering from underfunding, understaffing, and poor facilities. The life is gone and you have to let go - let the life end at its appointed time.
I'm a big believer in consistency. If you believe in the sanctity of life, don't contradict yourself by only professing your belief at convenient moments. And if you believe life should end at its appointed moment, this extends to opposing artificially extending the life as well as opposing prematurely abbreviating it.
(For the record, I'm torn on the issue of euthenasia. That's a matter of personal choice, it doesn't deny the rights or take the life of anyone else, and I recognise the arguments on both sides. It's an issue I will have to consider further, though it's fun to play devil's advocate in debates.)