(of course the recoil would be something pretty shocking - our butter-gun will probably not turn out to be a hand-held weapon)
So you agree with me you would have to throw the butter really fast.
Fry food in it for the victim and hope his arteries clog?
Is using a sling legal? How about a blow gun?
If limited to me, my victim, the butter, and nothing else in the world (no tools, no strips of cloth, no nothin’) I think I might be best off tossing the butter in his face to distract him while I attack him with my bare hands. I can do a lot more damage with my fists than I can with the butter, as far as I can figure.
Trinopus
I imagine a stick dropped from a plane would disintegrate before it ever reached the unlucky butteree… but if it worked… you could call it assault and buttery 
Gross (or grievous in Britain) buttery harm, even…
Sorry, sorry, the butter puns end here…
If we’re only allowed the butter and bare hands, then I think BraheSilver’s strategy is the most promising, if we’re allowed machinery, then accelerating the butter into some sort of high power dairy ion beam might be better.
To amplify what Mangetout said, look up “High pressure injection injury” to get an idea of what a high pressure stream of solvent or grease can do. With the proper equipment, a stick of butter is entirely suitable for shortening someone’s life.
Finagle
" a stick of butter is entirely suitable for shortening someone’s life."
Ghee that was bad!
“Boot to the head” All of you!
If you got lots of butter sticks, and melted them in one huge tank, you could drown someone in it, the density is much less than water so I doubt it would be possible to swim to safety.