Mirrim's life was generally traumatic, like any other Bossfights character. He'd been tortured and broken, physically and mentally. He'd had various psychotic episodes where he went on killing sprees. He'd had bad things done to him, and he'd done bad things himself. Mirrim was not by any means innocent. He was just the product of the Inkperials' horrible actions.
And then the war ended. And everything was quiet. Peaceful. Too peaceful. Because Mirrim worried. Every. Single. Day. He was paranoid. Paranoid of being attacked. Paranoid of going crazy. Paranoid of hurting his closest family. He would lie awake long past midnight, thinking about all the worst possibilities.
So he left. He simply couldn't live with this torturous conflict within his mind. So he wrote a note, and fled into the night, trying to escape the ghosts that would haunt him for the rest of his pathetic life. As he hopped into a ship, he rambled to himself about his plans.
"I'm gonna get out of here, be long gone before Oracle or Cassandra wake up. I'll be an explorer, and I'll help out as many people as I can. I'll forget the past, I'll leave it all behind, if I pull my broken shit together!"
And he did. Probably. Maybe?