I need to change one of my shocks and change all the springs to lowered ones.

I have bought a spring compressor ($21).

I want to know how dangerous it is in regards to using the spring compressor. Everyone is telling me its very dangerous, looking up on the internet i found out a 19yr old died, others saying that the spring should be chained down to something when compressing it, mate telling me not to do it at all..etc etc.

Is it true? Is it realy that dangerous?

Other then that i need to know how far to compress it, whats a safe amount and how the hell do i get the top bolt off. No*19 spaner doesnt fit in the cup to reach the nut properly and I cant use a torque wrench because something has to hold the shaft in place.