Why would someone suggest killing a spider rather than throwing it out of the window?

Care for the Lowest, by William Cowper (1731-1800)

I would not enter on my list of friends
(Though graced with polished manners and fine sense,
Yet wanting sensibility) the man
Who needlessly sets foot upon a worm.
An inadvertent step may crush the snail
That crawls at evening in the public path;
But he that has humanity, forewarned,
Will tread aside, and let the reptile live.
The creeping vermin, loathsome to the sight,
And charged perhaps with venom, that intrudes,
A visitor unwelcome, into scenes
Sacred to neatness and repose, the alcove,
The chamber, or refectory, may die:
A necessary act incurs no blame.
Not so when, held within their proper bounds,
And guiltless of offence, they range the air,
Or take their pastime in the spacious field:
There they are privileged; and he that hunts
Or harms them there is guilty of a wrong,
Disturbs the economy of nature’s realm,
Who, when she formed, designed them an abode.
The sum is this: If man’s convenience, health,
Or safety, interfere, his rights and claims
Are paramount, and must extinguish theirs.
Else they are all—the meanest things that are—
As free to live, and to enjoy that life,
As God was free to form them at the first,
Who in his sovereign wisdom made them all.
Ye, therefore, who love mercy, teach your sons
To love it too.

Martin said:

The house spiders and giant house spiders, if you put them outside will probably get in again.

I've noticed this before and now take them for a little walk outside before releasing them. In the spider catcher device, not on a lead! :)

The house spiders and giant house spiders, if you put them outside will probably get in again. In Spring and Autumn the males are searching for females, who tend to stay in one place. I try to trap them, but if they are in an awkward place, I use the hoover. I once heard one of the giant house spiders before I saw it, it was walking over a piece of paper. I think they are too large for me to be comfortable sharing a room with.

Me too. When I had cats they enjoyed stalking and killing spiders for me. Unfortunately I have had no spider killing pets to protect me for many years now.

I used to always try to trap and release them but they often escaped from me and then I'd be terrified because I didn't know where they were. One day when I saw a huge one on the floor I stamped on it in fear, and since then I haven't been so scared of them. They can stay outside or take their chances if they come into my home.

I'm terrified of spider, they make me feel physically sick, I can use the spider catcher thing we've got to catch some, but not all, sometimes I've had to suck them up with the hoover, 3 grown women all holding a hoover hose like firemen at a major incident, trying to get up the courage to get near enough to get it sucked up the hoover and then having to run screaming from the room after, all with pounding hearts and out of breath.

I can't stay in the same room as big spiders, I didn't have a bath for weeks, once just strip washes in the kitchen sink, I've stayed of of my bedroom and slept elsewhere and the living room.

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Well, i prefer not to kill them but I'm quite afraid of them.  There is therefore a risk that, when the spider realises I'm trying to catch it, it runs about uncontrollably or suddenly falls from the ceiling, leaving me in a bit of a panic (Good grief!  Is it in my hair or on my clothes now?).  If i'm panicking and rushing around trying to catch it, there's a chance i'll accidentally kill it anyway, either by stepping on it or by failure of one of the helpful devices I'm using to trap it (e.g. I had one of those little trapdoor boxes on a stick but the trapdoor often came down at the wrong time, guillotining the spider.  I've also got one of those brush devices but it's hard to use in corners or along edges - precisely where the spider will tend to go - and the spider might end up crushed or injured).  Basically I try very hard not to kill them but I find it all quite frustrating because it never seems like a case of just "throwing it out of a window" (how do i catch it, what do i pick it up with, how to a make sure it's not dropped on the floor on my way to the window, leaving me wondering where it's gone etc).  Baths are easy with a little toilet paper ladder to enable the spider to escape, but then, of course, there's the horrible feeling that the spider is now "at large" and this process can also take some time - annoying if someone's waiting for a bath. 

Maybe, then, Friend B has experienced some of this and therefore just goes straight to killing it.  I think it's a shame but I understand it.  :(     

Because people are lazy, or don't care about the biosphere, or are afraid of spiders, or any combination of those things.

If I need to remove one from my house I prefer to use the upturned glass and piece of paper method to try to avoid injuring it. I'll take it outside and gently release it into some suitable habitat.