Cho, when you leave your caps-lock on like that, it means that you're shouting at everyone. So unless you intend to shout all the time, you need to turn off your caps-lock key. And you also need to work on your punctuation.
As for why Sirius died, JKR could not possibly give us the feeling that a war is really taking place In this fictional universe unless we witnessed the death of one of the most beloved characters in the whole series. That, she knew would be the only way to bring home the true reality of warfare to her readers.
And also, consider this, if she didn't kill off someone we all loved very much very soon after the return of Voldemort, she wouldn't have been able to lend much credit to Voldemort being a truly evil and greatly powerful force in the wizarding world.
In order to truly enjoy a story you must feel as though you are right there with the characters when you read this. Jo Rowling knows this.

But that's just my opinion on the matter.