Sanctity of (a bug's) life

I'm not the most emotional of guys, but I don't like to kill things. I don't hunt or take potshots at squirrels. The one time I had to set mouse traps I felt guilty for weeks for murdering two rodents.

But I can kill bugs like popcorn. Spider? Smash it. Cockroach? Spray it. Mosquito? Mash it. All with glee.

Why is that? It's not because they're loathsome. I don't like rats or cats, but I wouldn't kill them if I didn't have to (no, I'm not killing cats).

Is it mammalism? Do I relate with mammals but not insects, so I can zap them? Is it cerebralism? An insects brain is the size of pollen, so they don't rate? Is it legism because two and four is fine but more is terminal?

I just don't know....