Sirius Black may have always been kind in the Harry Potter movies, but a moment of downright cruelty from the books was cut. Harry saw his godfather as a mix of a father figure and a brother, and was always eager for Black's advice and approval. Still, a few times in the books, Sirius proved that he might not have been entirely worthy of such admiration. Sirius Black's behavior could be shocking in Harry Potter canon, but it helped develop Harry's godfather into a far more interesting, albeit slightly problematic, character.
In the Harry Potter and the Order of the Phoenix book and movie, Sirius Black was confined to Grimmauld Place at Dumbledore's orders. Both versions of his character grew frustrated by this imprisonment, but Sirius' agitation in the book affected how he treated the people around him. Sirius felt he was being excluded from the battle against Voldemort and was eager to put his life on the line for those he cared about. For a time, Harry and Sirius could commiserate together over this, since Harry also felt disregarded by Dumbledore. However, this changed when Sirius' awful behavior was turned toward Harry.
Sirius Says Harry Is Less Like His Father Than He Thought In Order Of The Phoenix
Since Harry and Sirius both felt frustrated with Dumbledore for pushing them to the side in the Harry Potter books, Sirius thought he could count on his godson to help him break the rules. While talking to Harry through the Gryffindor common room fireplace in The Order of the Phoenix, Sirius suggested he come out to Hogsmeade Village disguised as a dog to visit Harry. Since Sirius Black was never proven innocent, Harry was too worried about his godfather getting caught to take the risk. He told Sirius no, and it did not go well. Sirius responded, "You’re less like your father than I thought. The risk would’ve been what made it fun for James."
After this discussion, Harry began to recognize that Sirius did not necessarily see him as his own person, but merely as a replacement for James. Sirius had not wanted to hurt his godson, of course, but he was particularly ill-equipped for being locked away after being confined in Azkaban for most of his adult life. Sirius missed his old school friend, and though it was not fair of him to do so, he allowed this pain to affect Harry. Sirius' immature and risky behavior meant his godson had no choice but to be the responsible one. He was forced to worry endlessly about his godfather, which made Harry easier for Voldemort to trick and manipulate.
Sirius Wasn't As Kindly In The Harry Potter Books As In the Movies
Sirius Black had a good heart, but he was not nearly as perfect as the Harry Potter movies portrayed. While the live-action version of his character gave Harry pep talks about the differences between good and evil, the other spent more time encouraging Harry to ignore Molly Weasley's rules. This, of course, set Sirius at odds with Molly, to whom he was never particularly kind. Furthermore, Harry learned in Severus Snape's school memories that Sirius and his own father had been cruel bullies during their days at Hogwarts.
The Harry Potter movie version of Snape's interaction with the Marauders in The Order of the Phoenix was pretty tame compared to the books, where Sirius' insults were far more venomous and hurtful. Then, when Harry confronted Sirius about this, his godfather laughed it off as two kids having a good time. Harry Potter began to think the Marauders - Sirius included - were not the role models he had initially thought, and questioned whether he even minded that Sirius did not think he was like his father.
Sirius' Cruelty Toward Kreacher Contributed To His Death
Sirius' harsh words to Harry made the young wizard worry about his godfather, which meant that when Voldemort showed Harry images of Sirius being tortured at the Ministry of Magic, it was like salt in a fresh wound. Perhaps if Sirius had handled his relationship with Harry more maturely, his godson would not have been so easily manipulated. However, the Boy Who Lived was not the only person Sirius had been cruel to. In the chapters of the Harry Potter books, Sirius acted unnecessarily harshly to the Black family house-elf, Kreacher. After Sirius died, Dumbledore said as much, admitting, "Indifference and neglect often do much more damage than outright dislike."
Kreacher's involvement in Sirius' death was cut from the Harry Potter movies, but his part was much bigger in the books. The enslaved house-elf had betrayed Sirius to Bellatrix Lestrange and Narcissa Malfoy, who convinced him to tell Harry that his master had left Grimmauld Place. Because of Kreacher, Harry believed Voldemort was torturing Sirius, and his godfather was killed as a result. As Dumbledore explained, this might not have been the outcome if Sirius had only treated Kreacher with kindness in Harry Potter, proving, like Sirius once said, that it is how someone treats their inferiors that defines them.