Every time I tell my old room mate I killed a spider he says 'what did it look like'. I have never (EVER) stopped long enough to inspect a spider before I either sprayed it to death with hairspray, trapped it under a cup or squished it with a shoe. All I see is a dot with a bunch of legs running around and think 'GET IT BEFORE IT ESCAPES TO COME BACK AND EAT YOU LATER!!'. I blame stories of how many spiders a person supposedly eats in their sleep in a lifetime (I slept on, yes ON my chin for a lot of years as a kid because a friend told me this)
That being said, if I'm outside and see one in a web or climbing somewhere that it belongs, I'm completely OK with them and let them be. I think there's just something about having them in my house that freaks the crap out of me.