She didn’t “have to” kill Hedwig, or Fred, or Colin Creevey, or blahblahblah ... only really “had to” kill Voldemort.
Well yes and no. One of the reasons authors kill off character is to maintain the apparent risk level. If you create a story filled with dangerous situation and nobody dies by the time you get to the final show down most of your audience has lost their belief in the risk. You have to kill character periodically to keep the risk present in the audience’s mind. Joss Whedon talks about this in the commentary of Serenity, he killed Shepherd Book near the middle to establish that this plot was playing for keeps, he kills Wash near the end so that when the characters setup for their last stand the audience believes it really could be the last stand; in order to get the audience thinking that way he had to kill characters. JKR had to do it here, and because of how the series started (until Cedric byes it in Goblet death is something that doesn’t happen to good guys) she needed a serious bloodletting, she had to establish that this wasn’t the beginning where nobody died, nor the middle of the series where one character would die, this is the end of the series they could all die.