Sirius wished he could talk to his brother, explain to Regulus where he was going wrong.

He burned with resentment at the memory of Regulus crouching by Kreacher’s side, comforting the house-elf after Sirius had kicked the disgusting sneak down the stairs. Couldn’t Reg see that Kreacher was a horrid, cowardly little spy, always prowling around after Sirius to try and catch him doing something wrong? Watching Sirius so he could run back to the mistress of the house and tattle-tale, then stand by smirking with satisfaction as Mother punished Sirius?

Being kind to Kreacher just encouraged this loathsome behaviour!