Stella was seeing Jason. Karl got very jealous. I can't recall if he just wanted to cause her problems or kill her or alternatively be a hero and rescue her. He lit the fire, sunita caught him, and in the process she died and the fire got out of control.
He tried to frame Jason, 'cos Jason was going out with Karl's ex, Stella, at the time. Jason had done some electrical work in the basement, so Karl started a fire in the basement to make it look like Jason's shoddy work was to blame.
Karl thought the building was empty when he started the fire ..........
As gizmonster said. I remember more fully now that Karl didn't know Stella was there and sunita, had she escaped, would have been a witness to him starting it and she hated him. Karl later tried rescuing Stella.
Also, Sunita didn't die in the fire, instead was seriously injured. She was in hospital unconscious, when she came round Karl tampered with her breathing tube and that's when she died.