Well it was rumord that Diaz has Napoleon Complex and he had anger issues but what happend was that he ambushed the drug deal between The Forellis and The Vance Crime family killing two Forelli goons and Lance's brother and Tommy And Lance wanted revenge so they start to work undercover for him and when everything is going well Lance tries to Take out Diaz himself but ends up getting taken to the junkyard and tortured in which Tommy saves him and they end Up ambushing Diaz's mansion killing Diaz along with a shit load of goons. So overall, he's too paranoid and probably thought that with the Forelli's moving into Vice City that they were going to try and take him out and take over his Business. And yes Original GTA Master you are right.
How many missions do you do Donald Love in gta: 3?