The Harry Potter series taught us all about the magic and mystery of the Wizarding World. Beyond the witches and wizards that we spent most of our time with, we also met a number of other non-human characters that ended up playing huge roles in the books.
While the most popular house-elf in the franchise was the delightfully chipper Dobby (R.I.P.), we also became a little familiar with Winky - who belonged to the Crouch family - and the Black family's devout servant, Kreacher.
Kreacher may have been a dark, bitter, and sullen house-elf who made life more difficult than easy for Sirius and later Harry, but over the course of the books he slowly grew on readers.
He might have been stubborn and loyal to only pure-blood wizards, but once Harry put effort into getting past his tough exterior and took the time to be kind to him, he learned that Kreacher was a product of his environment and not all-around bad. In short, Kreacher was merely misguided and looking for affection from whoever would give it to him.
By the time Harry Potter and the Deathly Hallows came to a close, we knew more of Kreacher's backstory and he was able to redeem himself. Eventually he even led the Hogwarts house-elves in the fight alongside Harry, against the people he had served his whole life, to honor his late master Regulus Black, during which he shouted:
"Fight! Fight for my master, the defender of the house-elves! Fight the Dark Lord, in the name of brave Regulus! Fight!"
The Redditor lmeaac recently posted a thread that may pose an answer to why Kreacher was, for at least a while, the terror that Sirius thought him to be. After being raised in a pure-blood family and serving the Blacks, it's no wonder that he had a skewed view of the Wizarding World. Kreacher also suffered some serious trauma.
Overall, his nasty attitude can at least partially be blamed on the circumstances in which he lived, but what if something more magical also took its toll on the old house-elf?
Kreacher was forced to let Regulus Black die in his attempt to destroy one of Voldemort's Horcruxes. Regulus even drank the Drink of Despair, though he could have ordered Kreacher to do it, just because he cared so much. After his beloved master died, it's safe to say Kreacher was scarred. From that point on, he also came into the care of Sirius who, despite his many redeeming qualities, treated Kreacher horribly.
As we know, Regulus' last command for Kreacher was to leave him and make sure to destroy the Horcrux. Even given his house-elf magic, Kreacher was unable to complete his final task, which made him live nearly two decades thinking he had failed the last person to show him any compassion.
Let's think back to when Harry, Hermione, and Ron had the locket in their possession. Being in close contact with something so evil nearly tore their entire friendship apart. What Imeaac proposes may have been the case is that the Horcrux amplified Kreacher's already bitter disposition.
Imagine that the evil of a Horcrux affects house-elves, who are much smaller in stature, amplifying the negative feelings the same way Butterbeer would to their systems (making them much more intoxicated than a wizard). Since Kreacher had the Horcrux in his care and kept it close to him for about 18 years, it's no wonder he always seemed so unhappy.
Once the locket is removed from the 12 Grimmauld Place, he grew much kinder towards Harry, and Ron, even having a new appreciation for Muggle-borns after knowing Hermione. In the end, maybe all Kreacher needed was some love, appreciation, and to get that Horcrux locket the heck away from him.
This may be looking a bit too far into Kreacher's character, and there's no way to know whether this is the case without J.K. Rowling confirming. But for those of you who love all things Harry Potter and any S.P.E.W. members, perhaps this fan theory suits your fancy!
Thanks again to lmeaac for bringing up such an interesting theory!