Flies are quite degenerated, devolving creatures that attract physical and psychic, astral filth. I personally do not want them in my home. But if you want to abstain from killing flies, that is your personal choice.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"