why the mistreatment of Sirius?

El gato keeps ratting me out and I've already had 3 lectures so I'm not writing about him today. I mean if I wasn't keeping an eye on him how could I be present to get the lecture? I've reread the first three Harry Potter books in French instead of reading something new and I still can't understand the way that Sirius was treated by J.K. Rowling. Hermione in the last book can catch fish and Sirius can't and has to eat rats? Sirius had money and a big house and needed company so who better for Dobby and Winky to work for? But no. Dumbledore went looking for one Horcrux. Who better than Sirius to go looking for Salazas Slytherin's locket but Sirius and Kreacher? That would have healed some of the enmity between them and Sirius would have learn't about his brother's sacrifice. So maybe Sirius could not use polyjuice potion because it would interfere with his Animagus transformation? But if the memories could be used by Dumbledore to find out what Voldemort was up to why couldn't they be used to prove Sirius's innocence? Even if the memories of the students could not be used because they were underage, Snape's memory could have been used if they did not want to trust Remus. Really I just don't get it and don't get me started on the numerous unnecessary deaths while people like the Malfoys did not even get their hair singed. Imagine killing off one of the twins instead of Percy? Jeez.