Normally I begin writing a song with just with aim to express something, and sometimes I don't know what I want to express until a sentence comes to my head that will sum up everything about how I'm feeling at the time.
I think you have to control the materials to an extent, but it's important to let the materials have a kind of power for themselves; like the natural power of gravity, if you are painting on a wall, it makes the paint trickle and it drips; there is no reason to fight that.
The commandment 'Thou shalt not kill' does not say it's O.K. to kill some people and not others.