Well, no one knows for sure if Tut was murdered or not. He was the son (Supposedly) of Akhenaten, who had the people go from worshiping many gods to worshiping one, the Aten, or sun disk. So he was very much disliked.
Tutankhamen's name was originally Tutankhaten. He changed it to Tutankhamun soon after coming to the throne at the age of 9. He also moved the capital from Akhetaten, the city of the Aten, to Thebes, the city of Amun and the former capital of the country. He also allowed everyone to worship the old gods again. All of this probably occured because of the power behind the throne. Tutankhamen was young and, of course, wasn't expected to rule the country by himself. Even older and more experience Pharaohs recieved help from Viziers and high counselors. So, who knows, maybe if the high officials hadn't interviened the cult of the Aten would have continued for some time.
Tutankhamen only ruled for 10 years. Since he died at such a young age, that of 18 or 19, many people think he was murdered. X-Rays of Tutankhamen's scull made in the 60's show that he suffered a possibly-fatal blow to the back of the head. As of yet we do not know if he died because of this. The Egyptian Government will not allow further x-rays or tests at this point in time.
But who are the possible suspects? Tutankhamen may have had many enemies, being the son of the most hated Pharaoh of all time. It could have been Aye, the former Vizier of Akhenaten and the Vizier of Tutankhamen as well. Could it have been Horemheb, cheif of the army? He had been a close associate of AKhenaten. he may have also had a deep hatred of the boy Pharaoh.
Then some think that it could have been his wife, Ankhsenamun. Somehow I see this as highly unlikely.
Aye is the most plausible suspect. He was in the right position, he was of the royal family (Father of Queen Nefertiti and Brother of Queen Tye) and so would have inherited the throne. Aye became the next Pharaoh after Tutankhamen. That's why he's the most likely suspect.
Horemheb had the chance, but didn't go for the throne when he had the chance, although he did become the Pharaoh after Aye.
Ankhsenamun wasn't mentioned after Tutankhamen's death, so no one knows what became of her, her mummy has as of yet not been unearthed or identified.
For more information on this subject, read, "The Murder of Tutankhamen" by Bob Brier and the most recent edition of TIME magazine.