20He's Insensitive With Words
Let’s get this out of the way: Sirius Black is a jerk in his teens. Sure, he’s endured a lot of hardship by the time he starts going to Hogwarts: his family is full of ardent blood-purity fanatics, and they disown anyone who disagrees. But even taking his questionable family
life into consideration, that’s no excuse to harm others.
Both Sirius and James call Snape “Snivellus,” making fun of his appearance and general existence.
In the same memory, Sirius remarks that he’s bored and wishes it was a full moon, and seems oblivious to Lupin’s gloominess. Even if they’re usually comfortable joking about it, Sirius doesn’t notice that his throwaway remark has taken it too far.