Q knows Picard's darkest secret - he never stopped being a Borg
Picard's time as a member of the Borg Collective was a huge tragedy, but Q confirmed that it never really ended - Locutus is still alive.
While Jean-Luc Picard is one of the most decorated and prestigious captains in Star Trek history, his addition to the Borg turned him into a deadly menace - and the godlike Q confirmed his Locutus Identity is still there.
In 2366, Picard was assimilated into the Borg Corporation, through whom the Corporation obtained vast data on Starfleet tactics and techniques, which they utilized during the Battle of Wolf 359. However, unlike other assimilated drones, Locutus is left with a small portion of his own autonomy and personality in order to best facilitate the assimilation of Earth. Queen Borg even wanted Locutus to be her equal, making him her perfect counterpart. Thankfully, this also makes it easier for the crew of the USS Enterprise-D to rescue their captain, cut him off from the Collective and restore him to his humanity.
In "Star Trek: Alien Spotlight: Q" - written by Scott Tipton and David Tipton, art by Elena Casagrande - the troublesome Q takes control of Picard and does a poor job running the ship in his stead , proposing the idea of removing Captain Locutus from within. Q describes Locutus as essentially another In Picard's mind, it's not just memories, suggesting that even after he left the Borg, the Locutus personality still retained a voice in his psyche. Ultimately, Picard turned down the alien's offer because he believed it was a part of him, but Q's revelation still has some very interesting implications.
Locutus Still Lives Inside Picard's Mind
Most recovery drones rescued from the Collective - such as Seven of Nine and her children - have had a harder and longer road back to humanity than Picard describes. Granted, the Starfleet officer hasn't been a part of the Borg for very long, nor has his personality been completely stripped away, but it's still unlikely that part of him will go away easily, or even at all. What was once a Borg will always be a Borg - a fact confirmed by the fact that drones still respond to him and call him Locutus, even years after his assimilation.
Of course, the ordeal was a great trauma and a grave violation of everything Picard held dear. It has been shown many times over the years that he never really gets over it, especially the parts where he is used As a tool to destroy Wolf 359's Starfleet Task Force, which is why Benjamin Sisko will never forgive him, however, Picard also learned to live with that part of his past and his psyche, allowing Q to move it It's not an easy thing to be merciful enough to lose a part of yourself.
Q What doesn't understand, and probably never will understand, is that humans need all parts of themselves and their past, good and bad. Locutus still exists inside Jean-Luc Picard, which helped him become who he is now, for better or worse, removing it like a defective part would end up making the unexpected goodness of Q in Star Trek Behavior with those of the Borg themselves.