Yes, it could kill someone. Absolutely. Though not accidentally. The staff will release a continuous stream of frost, but only as long as you will it to do so. A small dose of the spell could induce frostbite, freeze limbs, damage nerves... A more concentrated application of the magic could freeze a man solid or stop his heart.

[ This announced in a theatrically reverent manner. Magic is amazing. Amazing. ]

Aim the staff at the rock garden and will the power to leave it. It's that simple. Just a matter of your own intent.