Be warned, this is a branch of the story that will lead you down an evil path, killing everyone and helping the bad guys triumph. I hope you're proud of yourself. ;P
Once you get to Hollywood, follow the street you begin on to its end where you will find a television screen beckoning you over. Matthais is tired of fighting you, and before committing his time and effort to killing you, he is going to try an attempt at reasoning with you.
Save your game, just in case you pick the wrong dialogue options, then be civil with him. As long as you pick open minded dialogue options, and generally agree with his goals he will eventually give you the opportunity to join him, tasking you with the destruction of LA. Follow this quest chain to completion and the achievement should be yours. Show any signs of doubt in Matthais and his golden future along the way though and he will drop you like a bad habit. If you really wanna be evil, you gotta commit.