I'll concede they probably don't feel it like we or any of the complex beasties do, but they react to the stimuli. It's indicative they're not thrilled shitless.
They can't be thrilled either, though.
Tearing things apart just isn't nice.
This is such a weird statement. By utilitarian models of ethics, killing insects isn't even a moral question, since they don't have utility, so tearing things apart ISN'T nice, but it's not not nice, either. But as an emotivist, I would have to agree with you- if killing bugs makes you feel bad, then it's probably wrong. I think bugs are really cool, and don't like smashing them because it's like stepping on God's robots.