There are some extreme religious sects that take the sanctity of life very seriously. For example they will not pick fruit from trees to eat, but rather wait until it falls to the ground. Of course this type of view can only be held by a few people.
I like to feel that I am a little bit merciful. So although I had a number of small flies flying around my flat, I didn't immediately try to kill them. OK I tried with a fly swat and some spray. But my mercy was abused. I got tired of watching flies do doing dog fights at night, so I then I though no more Mt Nice Guy. I got a fly trap from an Euro store. As far as I can tell, it is just a piece of transparent sticky paper. I don't think poison is involved. Still not a nice way for a fly to die.
I have stuck the fly trap to a window for three days and I seem to have caught two flies. It looks a bit nasty though, much like putting the severed heads of criminals at the top of city walls.