the heart of an animal will never change.

Set is the very definition of ill-tempered, the times he has smiled can probably be counted on one hand. Most of the time, he's probably scowling and threatening some type of violence. Much more likely to get into a fist fight with you than actually talk things out. Being the god of violence and disorder lends itself to that.

People don't like Set and Set doesn't ever do himself any favors. He's infamous for his temper, killing slaves for not cleaning his sword properly or threatening to hurt anybody that doesn't treat the gods correctly. When Osiris is murdered and the murder is pinned on Set, nobody believes Set when he says he didn't do it. There is nobody on Set's side and people are happy to see him gone because they've wanted him gone since the beginning. Since he was a child, he's always had violent outbursts. At best, he's rude and at worst, you might end up either dead or hurt with a run-in with him.

However, once Osiris is assassinated and Set is executed despite his claims he would never murder his brother, he's brought back five years in the past and forced to confront his problems. Set is told that if he wants to change things, to save his brother's life, then he must change because he does love his brother, the king Osiris. Although Set himself does not know his own feelings or understand them because in Set's own words "I was never given love and so I don't understand it".

When Set was born, he burst out of his mother's stomach and killed her. Since that happened, his father absolutely hated him. As Set grew up, he was constantly put down by his father and deemed as somebody with very little worth. During Set's childhood, he desperately tried to gain his father's love while he only grew angrier that he never got it. Only Osiris ever showed him any kindness while Set threw fits and pushed everybody away.

By the end of the manga, Set finally sees what he has done wrong. How he has only ever been an ill-tempered asshole and must confront that his own situation is his own doing.