Put yourself in Sirius Black shoe. Your soulmate/best mate got married, had a kid and named you godfather. Life is cool, everyday a bit of risk here and there, but that's cool enough for you to live to see tomorrow.
Suddenly your godson had a target red on his whole body, and they had to go in hiding. You suspect that one of yours was the spy, and you know you were one of the suspect. Oh by the way, best mate and his family chose the Fidelius charm as protector and you to be the secret keeper. You'll keep the secret with your own life, of course, but nooooooo, that's too easy to guess. And you passed that role to the ultimately real spy.
Bam. Best mate got killed. His dear wife, same fate. But your godson survived. Just imagine the terror to arrive at the house feeling absolutely iced inside with dread, and then the sight there confirmed your dread.
Remember what Hagrid said? " White an' shakin'."
No less. He then argued with Hagrid over Harry and when the gamekeeper was adamant, Sirius handed over his precious flying motorbike. Maybe it's because of reading too much into it, but it seems to me at that moment Sirius initially finally became properly unhinged. He didn't care whether he had his bike or not. He didn't care later about the bodies around him that the traitor had blasted to frame him.
Remember what he did? He laughed. Maniacally. The epitome of the saying 'the point when everything falls apart that all you can do is laugh'.
Like George who will never really be the same again without Fred, Sirius probably was never properly healed and whole after James' death. The revelation that Peter was alive all along simply pulled the trigger to BE there, for Harry. Yes, guilt contributed to his prolonged stay at Azkaban, but revenge is what truly ended it.
Shame he never had the chance to kill Peter with his own hand.
Breaking free from Azkaban and reaching out to his godson, being on the run all that time and finally being there for Harry, had Sirius' scars been healed? Nah. Sealed, perhaps, but not entirely. We all see him hiding his mental breakdown at the mention of his best friend and Lily rising from the wands encounter.
'[...] Sirius grip on Harry's shoulder was now so tight it was painful.'
'[...] At this point, Harry found he could not continue. He looked around at Sirius and saw that he had his face in his hands'
One of the truest headcanons I've ever read is that 'Both Harry and Sirius saw each other as substitutes for James'. It was heartbreaking. The child that never truly met his parents and the man who longed for his dead best friend that looked exactly like his son. The cruel and ironic twist of fate.
Love you always, Padfoot 🐾
