It's bad to kill flies or mosquitoes?

Sin is to kill animals or insects for the sake of killing them. If, however, certain animals or insects life-threatening, integrity or health of people, they should be cut off. For as microbes must be killed, as well as flies or mosquitoes - carriers of viruses and diseases. If, however, we are in the open field and  in not any way we are affected by these insects, is a shame (sin) to
kill them.