JoJo said:
As far as we know, insects aren't sentient, so I can't see a moral reason why it'd be wrong to kill one. Personally though, I think bugs are pretty cool and so I won't set out to kill one unless it's something like a mosquito which intends to make me it's next meal.
So whether something is ok to kill depends on whether or not we consider it sentient, bearing in mind changing human views on sentience and limitations and bias of the human perspective.
Even accepting this terrible basis for life-or-death morality, what about evolution? Every living species could develop into a species that you would describe as "sentient". But if a creature is killed, he or she provides less aid in that development and any such death of an insect could be the very death that prevents such evolution.
Considering it anything other than wrong to kill creatures less powerful than ourselves with no ability to fight back is nothing other than a justification for sociopathy.
People might quickly change their tune if a powerful alien species decided humans weren't sentient (they had no proof of human sentience, which is all that matters to them) and randomly squashed humans. Sure, many of the ones squashed would be poor Africans that noone cares about anyway, but what if Stephen Hawking got squashed, or your favorite game developer? Your grieving would be no more witnessed by the Amazing Alien Species than your sentience.
It's part of sentience itself to fully appreciate the world and living creatures within it. Perhaps the aliens who would squash us have a bit of a point in not recognizing our own sentience.