Now you are angry at Diaz and it's time to repay him for what he did to Lance.
Meet with Lance on Starfish Island, once you are prepared (have full armor and full health and a range of weapons - do not buy a Luger as it will be replaced by a M4 at the start of the mission).
The mission itself is pretty simple, just follow Lance to the back of the villa and kill all thugs you encounter. Once you enter the villa, head up the stairs (beware for more enemies) to the second floor then head down the corridor and you will be above Diaz' office, looking down on him. Kill him to trigger the cut scene.
Rewards: $50.000; The Diaz Estate is now the Vercetti Estate.