Why did Rowling intend to kill Arthur in book five?

[Catlady (Rita Prince Winston)]

No: HPFGUIDX 180414

Now we know that the much-speculated character who got a reprieve was
Arthur Weasley in OoP, not anyone in DH. He was supposed to die from
Nagini's attack. That would show that war is hell. But I can't imagine
what the story would gain from it that's greater than or equal to what
it gains from Harry having saved Arthur's life.

As the book is, Harry's nightmare vision was not only true and
terrible, it was useful. That seems a MUCH stronger encouragement to
him to want to keep having the visions than if it had been true,
terrible, and useless. 'I don't like it much, but it's been useful,
hasn't it? I mean  . . I saw that snake attack Mr Weasley and if I
hadn't, Professor Dumbledore wouldn't have been able to save him,
would he?'

If it had been true, terrible, and useless, Harry might have listened
to himself when he said: 'And he might try and make me do things?'

(Surely the normal response to 'he has realised that he might be able
to access your thoughts and feelings in return — ' would be 'But I
don't know anything [so he couldn't find out anything from me].'
Harry's actual response presumably shows that he was vaguely aware of
the Dark Lord's intention through the scar link, all the more reason
he should have listened to himself.)

And he might have been less stubborn, maybe even willing to listen to
Hermione's logic about the Ministry of Magic not being deserted on
mid-afternoon of a weekday, if his previous attempt at saving someone
had turned out badly.

I think it would also mess up Harry's relationships with the Weasleys.
As it is, Mrs Weasley hugged Harry and said something about him having
saved practically the whole family by now (I guess that was in HBP
after Ron was poisoned) so that he felt welcome being part of the
Weasley family. If Arthur had died, even if the twins and Molly didn't
actually blame him, they would have been too grief-stricken to reach
out to him. He would have emotionally isolated himself from them (from
Ron and Ginny, too) because of his guilt, even greater than the guilt
he felt in the book when Arthur survived, and his reluctance to
intrude on their grief. That would have made it a lot harder for Ron
to remain the loyal friend who sticks by him, and for Rowling to get
him together with Ginny.

Mind you, the delightful Ginny of GoF became quite annoying in the
rest of the story. I could have done without her. But Rowling couldn't. 

I appreciated Betsy Hp's explanation of why Ginny  kept attacking
people by flying into them on her broomstick and casting bat-bogey
hexes; that whenever Rowling wrote of female sexuality, she wrote of
it as an attack, and Ginny's only purpose in the story is
romantic/erotic, therefore sexual. That explains why going down the
corridor hexing everyone you pass, just because you can, was a bad
thing when James did it but a good thing when Ginny did it.