In the intro to The Don Killuminati Kadafi says "Suge shot him".

Suge Knight told Tupac to take of his bullet proof vest because it was hot. Tupac always wore it and it could of saved his life if he did not take it off. Why wouldn't Suge want Tupac to wear it?

Tupac was about to leave death row. Tupac wanted his own record company called Makaveli records with the Outlawz. With Tupac gone Death Row would not make any money of Tupac. If they killed him they could make money from unreleased songs

Suge Knight and death row records were uncooperative with police after the killing. Suge Knight said he would not tell police who the killer was if he knew.

Kadafi of tha Outlawz saw who killed Tupac, and was going to tell the police but he was killed a month after Tupac.

Crips

A Death Row medallion was stolen from a member of Death Row, allegedly stolen by Orlando Anderson. Three months later at the MGM in Las Vegas Suge Knight and Tupac saw Orlando Anderson who had stolen the medallion and Suge and his entourage started to attack him. Death Row left before the police came. Later Orlando and the Crips were looking for Tupac as he was going to Suges club 662. A car pulled up next to Tupacs and opened fire killiing Tupac. Orlando Anderson was later arrested.

Mobb Deep

Mobb Deep and Tupac dissed each other on record. Mobb Deep were in California when Tupac was shot. This is from a Mobb Deep song.

"you tried to test my patients
                                       but i got somethin for ya
                                       i creeped on ya weakness
                                       then came to california 
                                       my crew loadin nines 
                                       screamin now it's past ya time 
                                       on the border where we stood 
                                       anxiously awaiting your ritious crooks
                                       H slammed the door on the caddy 
                                       blaze the blunt & load the daddy
                                       drove past ya fuckin sign
                                       what was yours suddenly became min
                                       shattered the window with the nine
                                       killin bitches is a must, not a crime"
                                       

Black Mafia

The Black Mafia said they shot Tupac in New York. Black Mafia wanted Tupac to sign to Bad Boy but Tupac refused. Tupac was to die in New York for saying no. Tupac could have been killed in Las Vegas because he was not killed in New York.