The ability for something to do damage depends on it's momentum, and that is highly dependent the mass and centre of gravity of the object as it applies force against something. You do a 1kg drone a serious disservice comparing it to an iron mallet. A drone is a complex shape and its weight is distributed over a comparatively large area. While crashing it will be taking a complex path to the ground. When comparing the likelihood of it killing you remember it's far more likely to be the same as a falling iron mallet, horizontally, hitting a person by the handle rather than the iron centre part. You're far more likely to get a bruise from the mallet's handle.
Not really. The primary cause of momentum is the acceleration from gravity which is essentially constant for a falling drone, air resistance is just a counter-force proportional to the velocity squared. That is to say, the reduction from air resistance only becomes significant when you're falling very fast. A skydiver has a terminal velocity of about 195 km/h, but already a high diver from a 30m height will be close to 100km/h. I know my buddy's DJI Phantom goes up to 120m by default, at that height you'll be at 90% of terminal speed. Unless it's extremely small or light like a feather or a coin, any object dropped from a big height is a lethal weapon. When the rotors stop the body of a drone is quite compact and holds a heavy battery, it'll fall pretty much straight down. You can see a good example of a drone coming down after a complete power loss here.
As for the actual impact the higher the velocity, the less it matters how it hits you or how soft or elastic it is, a stick of butter dropped from a skyscraper will still hit you like a brick. We have a pretty thick skull but the sheer momentum will be like being hit in the head with the full swing of a baseball bat, your brain will bounce around in there like the ball in a pinball game. Sure there are arguably worse things to be hit by, but for the most part we secure tall objects. There's falling coconuts and big icicles and such we can't fully control, but drones are a new threat in the hands of idiots. They're not very dangerous, but in the hands of idiots many things are dangerous.