In Harry Potter and the Order of the Phoenix, Kreacher passes information about Sirius to Narcissa and Bellatrix, that leads to the events in the Department of Mysteries.
The reason for this, from Harry Potter and the Deathly Hallows,
“Sirius was horrible to Kreacher, Harry, and it’s no good looking like that, you know it’s true. Kreacher had been alone for a long time when Sirius came to live here, and he was probably starving for a bit of affection. I’m sure ‘Miss Cissy’ and ‘Miss Bella’ were perfectly lovely to Kreacher when he turned up, so he did them a favor and told them everything they wanted to know. I’ve said all along that wizards would pay for how they treat house-elves. Well, Voldemort did . . . and so did Sirius.”
This would mean that they had been kind to him in the past, before Kreacher came to know of secrets of the Order.
The Malfoy family, of which Narcissa was a part, treated their own house-elf, Dobby, very badly, and in general, house elves are treated beneath the level of wizards.
So, why were Narcissa and Bellatrix kind to Kreacher, to an extent that he wanted to go to them?
If, for some reason, they were not kind to him before he became a source of valuable information, why would they try to get it out of him kindly? Doesn't it seem against their nature of treating house elves in general?