The fire broke out almost simultaneously in eight different locations in the hotel premises, leaving one person dead, and 11 persons injured.
Pongsakorn Boonaram was found dead in the bathroom of a guest room on the third flood.
Bangkok Metropolitan Police chief Kamronwit Thoopkrachang said Mr Pongsakorn had been in all of the areas which had burned, according to an investigation both from closed-circuit security monitors and eyewitnesses, while a cigarette lighter was discovered in the room where the victim died.
Meanwhile, Mr Pongsakorn left a message at his home, describing his strong attachment to the hotel and that he was ready to die with it.
He was having conflicts with the hotel, police said, without specifying whether it was with management or fellow employees.
The hotel will be searched again for more evidence, while the cause of the fire is still to be confirmed by Forensic Science Police.