In DH Hermione says that Kreacher doesn't think like that, he parrots the beliefs of people who are kind to him like Mrs. Black, Regulus and at the end of this book Bellatrix and Narcissa, and that brutal treatment isn't so uncommon for house elves so Kreacher didn't care so much about it. What happened to Kreacher clearly bothered him, but I think more so because telling Regulus led him to make the decision he did. I find it hard to accept, but then again, I'm not a house elf.