Why is this video of a G36 Bonanza taking off so scary?
Because those trees will kill them

If you look at this document regarding the Bonanza G36 performance you will note that the lowest stall speed written is 61 kts. 61 kts is 112 kph or 71 mph. We can assume they were going a little bit faster than that but let us say 110 kph / 70 mph. 

In that clip, they are just barely keeping airborne. This means the aircraft is extremely sensitive to disruptions. Any loss of power — even partial — or getting knocked back by a collision and thus losing a few knots or upsetting the aerodynamics of the aircraft, is very likely to send it crashing into the trees. 

Going 110 kph / 70 mph into a tree is rarely survivable even in a car, and then you have to remember that cars these days are built to withstand crashes while at least keeping the occupants alive, but that assumes crashing into something wide, that distributes the forces over a wide area, and that gives way, like another car. Trees fail that on both accounts. 

A G36 does not even come close to any such structural integrity in case of a crash. The cabin will crumple like figurative tin foil, sending the occupants smashing into the trunks of the trees, or throwing them clear of the aircraft only to then smack into other trees or the ground from several meters up and at a high speed. And that is going to be fatal. 

Hence, this is scary because if they clip a tree they are very likely to die.