If there is an opposite to the loveable house elf Dobby in the Harry Potter series, it has to be that real jerk of a house elf, Kreacher. Kreacher worked as Sirius Black's house elf and pretty much hated anyone who wasn't a pure-blood wizard, making him a wizarding-racist. Now one fan is theorizing that maybe Kreacher's less-than-kind disposition was not his fault after all.
I think that [Kreacher] is a prick in general since his first appearance because he was in possession of the amulet horcrux for a long time which is revealed in The Deathly Hollows Part 1. It is shown that the horcrux makes Ron very jealous and angry when he later has possession of the horcrux for only a short time. Therefore, when Regulus Black gave Kreacher the horcrux to hold onto, it turned Kreacher bad. Regulus was a young adult when that happened and so it makes sense that Kreacher would be mean to Sirius, the younger brother since Kreacher had the amulet before Sirius could remember Kreacher. The horcrux amplified the negative emotions that Ron had and similarly it makes Kreacher hate mudbloods even more and disrespect Hermoine way more than he would have if he didn’t have the horcrux.