Kages in the Naruto series are some of the most powerful shinobis who are quite important to their respective villages. 'Hokage' is the title given to the strongest shinobi responsible for taking care of the Hidden Leaf Village. So far, there have been seven Hokages, the most recent being the titular Naruto Uzumaki.
The anime's fanbase has noticed a rather odd trend that puts most Hokages in a bad light. If we take a look at some of the characters that posed a threat to the entire village, there were quite a few who were either students of a Hokage, or students of a shinobi that went on to become a Hokage.
Naruto: Taking a look at a few Hokages and their students that turned out to be major antagonists
The Third Hokage, Hiruzen Sarutobi trained three shinobis that went on to be known as the Legendary Sannin. Among them was Orochimaru, who was a major threat to the village and was responsible for Hiruzen’s death as well. He constantly attacked the Leaf Village and would go to extreme ends to achieve his goals. As a teacher, Hiruzen should have been able to identify Orochimaru’s lust for power and immortality.
Similarly, Minato, the Fourth Hokage, had a student who ended up becoming a threat to the village, who was none other than Obito Uchiha. Minato was indeed a great shinobi and was quite compassionate as a human being. But he couldn't stop Obito from becoming one of the biggest and most powerful antagonists in the series.
Madara orchestrated a chain of events that warped Obito’s perspective of the world. However, Minato did believe that he wasn't a great teacher and his presence during the mission involving Kakashi, Obito and Rin could have altered the course of the show for the good.
That being said, there is enough evidence to suggest that Kakashi Hatake did not do a great job when it came to mentoring Sasuke. Before we go ahead, it is important to state that the former was an exceptional shinobi who shaped the latter’s abilities. However, the role of a mentor extends beyond the confines of combat abilities.
The first issue with Kakashi's approach was his clear interest in Sasuke Uchiha alone. While the former was responsible for training Team 7, most of his efforts went into training Sasuke, with Naruto and Sakura being ignored to a considerable degree, and that is now how one should be training a squad containing 3 people.
After Sasuke left the Hidden Leaf Village, Kakashi focused on training Naruto instead of training him and Sakura together. That being said, there was one interaction that served as proof of Kakashi’s inability to be a good teacher.
Given his background, Kakashi knew exactly what it was like to be in Sasuke’s shoes. He lost everyone he considered dear, and therefore, one would expect him to have the right words for Sasuke, who was just a child who witnessed the death of his entire clan at his brother’s hands.
Instead of being more empathetic, Kakashi tied Sasuke to a tree and asked him to “forget it”. He further advised the young shinobi to refrain from entering the path of violence and revenge. Given Sasuke’s age and maturity, he could have been more patient in this situation and handled it better. While plenty of people love Kakashi's speech, it is important to understand this from a child's point of view.
These are just a few reasons why Kakashi is considered a bad teacher in the Naruto series.