[schoen]

The aliens might be equally surprised that many of our medicines are also poisonous to us. Yet (as someone put it to me when describing anti-worm medicine that his dog was taking) they're more poisonous to our pathogens or parasites than they are to us. For example, ivermectin, which that dog was probably taking, is poisonous to mammals but much more poisonous to their parasites, while terbinafine is poisonous to humans but much more poisonous to fungi that infect us.

For all of our medicines, we or our doctors have to make a judgment about whether the side effects are worth it for the therapeutic value that we get. If we said that we would never deliberately use any poisonous substances, we might have to throw away most of our pharmacopoeia.

You can try to make an argument against pesticides on the basis that their side effects aren't worth it (and I prefer organics myself), but I think the fact that pesticides are "poison" doesn't get you there by itself.