Why did this happen in Harry Potter?

i was the most sad when dumbledore and sirius died and Im pretty sure dobby's a dude and i think that she had to kill some characters off even some of the most important ones because on the story wouldnt be intresting and two if u have ever read the house of night series its like what nefret did to zoey if u havent voltemort and other sources are trying to get harry alone so he wont be as strong.