(A quick 411 on house-elves: they generally work as servants to the older wizarding families. They are bound to the masters, to whom they have absolute loyalty. They can only be freed if their masters present them with a gift of clothes. So far in the series, we have gotten to know two house-elves, Dobby and Winky. Now, in Book 5, we meet a third: Kreacher.)
House-elves are a tricky subject in the wizarding world. Hermione's ham-fisted efforts to free them through SPEW (Society for the Promotion of Elfish Welfare) in Book 4 seem naive, since 99.9% of house-elves don't want to be freed. Sure, they may seem like slaves to us, cooking and cleaning and generally waiting on their masters' every whim. But they claim to like it that way. It's a tricky problem that comes up yet again with Kreacher, the Black family's house-elf.
Kreacher is bound to Number Twelve, Grimmauld Place. When Sirius's mother died, Kreacher was left alone in this dark, rotting home with no company except the shrieking portrait of Mrs. Black. Some time in that decade alone, his loyalty went perverse. He admires the Black family (who were all Dark wizards) hugely, and he spends a lot of time squirreling away relics of the family in a little nest he sets up under the boiler in the basement. He also never seems to realize that people can hear him mumbling aloud, as he wanders through the halls of Number Twelve saying things he has heard from his now-dead masters about blood traitors and filth.
The thing about Kreacher is, he has been twisted by the family he served. And when Sirius arrives in Number Twelve, Sirius often takes his bitterness at his horrible family out on Kreacher. Let's take, for example, one exchange between Sirius and Kreacher early on in the book:
"My mother didn't have a heart, Kreacher," snapped Sirius. "She kept herself alive out of pure spit."
Kreacher bowed again as he spoke.
"Whatever Master says," he muttered furiously. "Master is not fit to wipe slime for his mother's boots, oh, my poor mistress, what would she say if she saw Kreacher serving him, how she hated him, what a disappointment he was —"
There is mutual loathing on each side of this master-servant relationship. Sirius despises Kreacher because Kreacher loves the Black family that Sirius ran away from at sixteen. Sirius spends most of his time mocking Kreacher for his devotion. But Kreacher is devoted by nature: he's a house-elf! It's what he does! He can't help what he is. So Sirius's constant put-downs only make Kreacher more aware that his current master is not at all like Kreacher's previous masters. Yet, Kreacher is bound to serve Sirius, no matter how unwillingly. He really is like a slave.
Kreacher's awful relationship with Sirius drives him to terrible things: one day, Sirius shouts at Kreacher to get out. And Kreacher takes this command literally to mean out of the house. Kreacher leaves Number Twelve and seeks out Bellatrix Lestrange, Sirius's cousin. Bellatrix Lestrange née Black is the only surviving member of Sirius's family whom Kreacher actually respects. With Bellatrix's help, Kreacher forms a plan to decoy Sirius to Buckbeak the hippogriff's room on the day that Voldemort lures Harry to the Department of Mysteries.
By injuring Buckbeak so that Sirius has to tend his wounds, Kreacher can make sure Sirius is out of the way when Harry Firecalls anxiously looking for Sirius. Kreacher is the one who confirms (falsely) that Sirius has gone to the Department of Mysteries. Later, as Professor Dumbledore is explaining all of this to Harry, he tells Harry that Kreacher laughed as he recounted his vile plan to Professor Dumbledore. In other words, Kreacher feels no remorse for bringing about the death of the last of the Black line, the family he has served his whole life.
The house-elf/wizard relationship may not be as black-and-white as Hermione paints it. But it does lend itself to these kinds of abuses. So even if Hermione is a little awkward and clumsy in her efforts to campaign for house-elf rights, she does have a point. As Professor Dumbledore says, "Kreacher is what he has been made by wizards [...] And whatever Kreacher's faults, it must be admitted that Sirius did nothing to make Kreacher's lot easier —" (37.114). Kreacher is a nasty piece of work, but he has also been warped by a lifetime of slavery to a cruel family.