Very true Ratty. We don't know the full circumstances of the situation so it could very well have been used as a reason to explain (excuse?) murder as you suggest. I tell you what though, whoever did that did it with quite some force, the skull is astonishingly tough so to push a triangular blade like that so far into a head would likely have needed a good deal of effort. In other words there's a lot of aggression behind it.
@ silicone : getting stabbed in the head is a rather extreme way of being converted to vegetarianism!