I like Kreacher, but I can see his faults. And I can also see how to be evil to him. I'll refrain from bodily harm this time, though.
Kreacher stalked the halls of the House of Black, muttering, "Mudbloods… besmirching the noble house of Black, my mistress's horrible turncoat son…"
Suddenly, Fred and George appeared beside Kreacher and yelled, "WE HAVE PROOF! The House of Black is a sham, they're all Muggleborns! Look, Kreacher!"
They showed the poor house elf the Black family tree and the blood status under each name, written obviously in blood. Family trees were only determined by writing on a charmed parchment with your own blood, so Kreacher knew that it was true.
Kreacher stared. Suddenly, he burst out into sobs. "The great house of Black is a lie! Kreacher has been working for Mudbloods!"
Fred and George patted him sympathetically on the back, then Disapparated back to their room. Once safe, they shared grins and high fives.
"Never knew he would fall for that, Gred."
"Just a little bit of ground up Blood Pops and water, Forge."
"How correct, genius brother-o'-mine."
"I got the brains, you got the looks."
"And we're both happy with that deal."
"Too right, Forge!"