A possible explaination as to why Hiei acts so differently in the beginning of the series until we see him again in th 4 Saint Beasts arc. We all know it's because he was supposed to be a one-off character but there is a possible in universe explaination not many people have thought of (at least it seems that way).
*Before Yusuke ever met Kurama and Hiei...*
Hiei was sitting at the base of a tree outside of Kurama's home. "Hiei?" Kurama asked, leaving for the hospital to visit his mom that morning, "What are you doing here?"
"I still can't find her, Kurama," Hiei complained, "I've been here for months and even with my jagan eye I can't pinpoint her location." He glared at nothing.
"I don't have time to worry for you," he said calmly, "why don't you try King Yama in spirit world?"
"What?" Hiei asked.
Kurama explained, "Well whenever demons enter the living world they usually know where they are in case they cause trouble. I doubt your sister is causing an issue but they probably know where Yukina is anyway." He started to walk away.
Hiei stood up and said, "Perhaps it was a good thing I came to see you." He disappeared. Kurama hurried to the hospital.
Hiei entered spirit world and walked into King Yama's office. The floor fell from under him and he suddenly in a dark room, surrounded by ogres. "What have we here?" King Yama said, "Just a D class demon."
"What the hell does that mean?" Hiei demanded, drawing his sword. He could tell he was surrounded even in the darkness. "Where are you?"
"He's the perfect case," King Yama said, "chain him down."
Ogres lunged at him in the darkness with his speed he cut them all down, causing blood to spill everywhere. In an instant they were seemingly all dead. More spilled out of the openings of the walls and poured into the darkness. As he cut more down his sword, it came out of his hands and flew across the room into darkness.
"Dammit!" Hiei shouted. The next thing he knew his arms were pinned against the wall, and his wrists chained. Because the jagan eye weakened him so much his flame wasn't strong enough to melt the metal cuffs.
"You have the jagan eye," King Yama said, still not revealing himself, "very interesting. I have the perfect task for you."
Hiei demanded, "What the hell do you want? Show yourself!"
A bright light lit up the room. It was surprisingly empty, leaving no trace of the countless ogres he had just killed and speakers were on the walls. The light circurled around the room on a track. King Yama's voice said, "You want all humans dead."
"What?" he asked, "I couldn't care less about the humans."
"You want all humans dead," King Yama repeated for several minutes that turned into hours. The light slowly began to hypnotize Hiei unwillingly. He could feel himself feeling relaxed.
Hiei's pupils shrunk. He couldn't think clearly anymore. "All of the humans deserve to die," he said without emotion.
"Yes," King Yama said, "the vault. It has the artifacts of darkness. With the shadow sword and your jagan eye you can control hundreds of humans. They'll be your slaves. What do you want?"
"Every human dead," Hiei replied calmly. His cuffs were released.
