Soga has a cruel and violent personality, and likes to humiliate and attack people. This stems from the fact that he has been discriminated against due to his actions and his Quirk. He seems to think that everyone around him sees him as some sort of monster, and has developed a love of making other people suffer in place of him due to these circumstances.

He shows some signs of mental instability, as shown when he sees everyone around him with a mocking, condescending smile, and laughing at him, no matter who they are and what they are actually doing, suggesting that he is unable to separate reality from his perception of the world. It is 

suggested that this perception of the world is what makes him so violent.

In spite of these flaws, he shows solidarity with his friends, who he takes a leadership role for. He feels kinship with others who have been looked down upon due to their Quirks. He shows a strong sense of loyalty and care towards them, such as when he went to face Kuin alone and warned her to leave his friends out of their conflict. He also is very defiant when he knows he is being played, totally trashing a selection of Trigger drugs because he refused to aid in Kuin's plan in any way.