Blame this.
Magiki Sfaira
Well, it’s like this. Back in the fourth century BC, one of the proto-mages that the Greeks had hanging around back then did a favor for a friend of his and enchanted a sling-stone to ‘kill the enemy its user chose.’ Unfortunately, the spell did not do anything to actually help with the aiming of the sling-stone in question; the first time it was used, the slinger missed his target completely. And then apparently promptly forgot about it, except to possibly mildly complain to his magician friend.
The Magiki Sfaira has been the thaumaturgical equivalent of the Goose That Laid The Golden Egg ever since then. The original mage’s notes have been lost, assuming that he ever wrote any in the first place, and nobody else ever learned the enchantment that he used. Which is a real pity, because black-box research on the Magiki Sfaira suggests that said enchantment was a masterpiece of efficiency and potency. Even today’s state of the magical arts would probably still be improved if the spell was finally reproduced — to say nothing of the prestige that would result from finally cracking the code — but the item is still ‘stuck‘ in active mode.
This is important because while a good mage can usually trace out at least the broad outlines of a spell’s magical architecture when it’s in passive mode, when it’s in active mode the task becomes… problematical. It’s like trying to do repair work on a fission plant while it’s running. People get wary if somebody even tries — and the more they know about the subject, the more wary they get.
Needless to say, the easiest way around this problem is simply to figure out how to use the Magiki Sfaira to kill its original target. Or perhaps that should be ‘easiest,’ given that the target is currently unknown, has been dead for over two thousand years, and probably not particularly deserving of getting his head bashed in with a rock? Any number of mages have certainly tried to finesse the situation over the years: none have succeeded.
But some will still try. Or pay well for others to try. And, hey, what one mage can do another can undo, right?