Well, if a Leper Magus dies for their magic, they get 15 vis out of it. Of course, as far as I can tell, this is at least partly Divine in nature (as it's based in leprosy).
The easy way to do this is Chthonic Magic. If you want to allow death to release vis in the casting of a ritual, that sounds like a custom Mystery Virtue that allows killing someone as part of a Ritual spell to provide vis for the casting.