Though he goes on about justice on a regular bases, his view of it is incredibly self-indulgent and one sided. As Naofumi and later Rishia both put it Itsuki's justice is not the same as someone else's justice, and there are as many types of justice in the world as their are people.
After the Spirit Turtle incident, his comrades all abandon him and he's reduced to a hollow shell of a man who goes around fighting in tournaments solely to gain praise from the crowd. He's later approached in this broken state and taken advantage of by Mald and Malty, as well as the revolutionary faction, and is given a Curse Series weapon based on Pride.
Under the influence of the Curse Series Itsuki wishes to bring everyone in the world to understand him and his justice, via brainwashing arrows, and goes completely off kilter, trying to outright kill Rishia when she said his stance on justice was utterly mistaken. He believes Naofumi is evil for taking slaves and turning away people who he could supposedly revive from the dead, because Malty tricked him into thinking the latter was possible. Naofumi goes on to remind him that slavery is not illegal in Melromarc, and everyone else present goes on to state Naofumi's slaves treat him like he's their mother more so than their master, and that the revolutionaries had brainwashed said slaves and were using them for their own convenience. His own Legendary Bow, if Rishia is to be believed, feels he's gone too far and gives its power to her specifically to help her bring a stop to it.
After suffering defeat at Rishia's hands he's become a total "yes man"; he would dance, handstand, or execute himself if ordered, and cannot tell lies. He's left in Rishia's care until he recovers from this state, though Naofumi is brought to wonder how she's treating him and has discovered that they sleep in the same bed.
Like the other three heroes he believed the world they had come into was a game, but he goes further than they do and believes Naofumi has met god I.e. the game master, and received powers from him to explain his superiority. Even after becoming a yes man and having everything shattered, he still feels Rishia being stronger than him has to be a cheat. Like the other heroes he thought he was immortal as well, since the "game" wouldn't let them die.
Post Curse The curse Itsuki received was a curse that effected thought process. His decision making power was nulled. The curse took away his will in itself. Itsuki was incapable of lying. Itsuki said the first thing that popped into his head. He couldn’t make up lies.