The blood of the murdered child will never wash off his hands.
Of course, this is a metaphor. He has no hands and there was no blood. A shuttlecraft accident, explosion in her impulse engine, quick and clean. He tries to tell himself it was over so fast, she couldn't even have known she was dying, but he knows better. She was a Q, albeit an unworthy one who'd rejected the Continuum and needed to die. She would have known what suddenly being cut off from the powers she'd been trying so hard not to use meant. She'd have had a nanosecond to understand her own impending death, but for a Q, a nanosecond was entirely long enough to feel terror, and regret.
Picard knows it was him. He hasn't been back there. The self-righteous posturing, the declaration of superior morality from a being with the life span and intelligence of an insect-- he just can't deal with that right now. He doesn't want to hear it. He doesn't want to see Picard's face, feel the brunt of his contempt. It was necessary but try convincing a human of that. Such soft creatures, so enamored of their own personal freedoms that they run around imposing their view of freedom on the universe.
With power comes responsibility. They took away his own powers to teach him that lesson. He wasn't happy about it, but the girl broke the law. She had to die.
He loses interest in Vash. All she ever was to him was a vicarious connection to Picard. Now Picard has been poisoned against him by his own actions, and he knows better than to think he can fix it. He dumps her on Earth, because he did promise Picard he'd make sure she wasn't hurt by being with him, and then he takes off. His obsession with humanity has curdled around him, turning bitter and poisoned for being truncated. He cannot go back and study the humans because he can't face Picard. He can't face Picard because he killed the girl. He killed the girl because he had to.
(because they'd have taken his powers if he hadn't, and he can't face that, never again)
He doesn't cross humanity's path again until some idiot accidentally releases Q from the comet they'd imprisoned him in. Q is still insisting on the right to die, and refuses to go back into protective custody. The human captain suggests she could rule on this issue, as the two Qs are evenly matched and without some arbitration they can only dance around each other, running and chasing, catching and escaping, for the rest of eternity.
He is not interested in listening to human starship captains posturing as if they have moral high ground over the Q. And after being forced to kill the girl he will not have the death of any more Q on his hands. He calls in the Continuum, and they recapture Q together and imprison him again.
The combined voices of the Continuum praise him. He's finally learned to work with the group, not against it.
He wishes the thought didn't make him so bitterly angry.
His companion calls him boring, the ultimate insult, and refuses to spend time with him anymore. His former friends say he's sold out. They're all absolutely correct. He's bored and boring, nothing in the universe engages him anymore, and he can't very well present himself as the defiant rebel angel when he's the Continuum's bully boy and assassin now. They told him his personal judgment was suspect, given his history, and let him exercise no leeway with it. They own him, but then, they always did, and he's just realizing after a billion years that fighting the truth is meaningless.
He doesn't think of the Continuum as "us" anymore. Always "them."
He stares into eternity, straitjacketed by the twin lashes of fear and boredom, and understands why Q might have wanted to kill himself. But a comet's not any better than this, and if he were going to disobey and get himself killed, he should have done it before he murdered a Q child to save his own life.
A mortal has death to look forward to, at least. Eternity sprawls out before him and he knows it will never, ever get any better than this.