I think that means that he has the choice to believe what they are telling him, and live thinking that he was a monster who killed his own wife. Or dying as a good man in the sense that he rejects what they are saying to him, and sticks to the fact that he was a good husband and “died” trying to blow the lid off Shutter Island. But then again Scorsece did an unbelievable job really leaving the opinion up the to audience.