If anyone is interested in something very like this idea in fiction form, C.S. Friedman's "Feast of Souls" has a world where magic requires life force. Witches do small spells and die young, using their own life force to power them. Magisters do greater magic, and kill another person every so often to power their magic. It's an interesting take on the idea, I think (though not her best series), and has a magister as the (very, very grey) protagonist.
Would I? No way in hell. Immortality scares me, to be honest. How long before you start looking at people around you the same way a human would look at ants?