Nothing is ever so black and white. I'd argue that there is a difference between killing an insect and killing a human. The difference, in my mind, is that we think an insect lacks the same capacity for experiencing pain.

Edit: In the US, a lot of how cruelty is valued is based on how alike the animal is to us. Mammals get more attention than birds, which get more attention than fish. There is a hierarchy and I think it's based on how closely the animal resembles us, a part of which is the capacity for pain and suffering.