1- Insects are animals, so you're committing animal cruelty unless you're doing it for self-defense.
2- Insects were here first, by over 300 million years. Technically, we're on their turf.
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3- Other things eat insects, like birds and lizards.
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4- If you kill a good insect, you'll wind up with more bad insects than you can possibly kill.
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5- Many insects do important jobs like pollination, decomposition, honey, beeswax, etc.
6- Live and let live, aye?