Now chances are the DM just wanted to play paranoia, and he told everyone this.
In this case, the only way to survive is to strike first. Therefore, wait for your briefing. Once the briefing is over, the first person to mention the first and last mission objectives exclusively in the same sentence is the traitor. Kill the person directly to his left using faulty equipment and bad information.
After he is dead, remove his power armor, weapons and face and assume his identity. While posing as him, summaraily exicute the traitor and the person standing directly to his right.
Then turn to any survivors and reveal that you are actually a daemon who orchestrated the entire incident.
Make your way to the reclusium and strike down any who follow you with what ever weapon is stored in it's center. Be sure to decapitate the Apothecary. (If you are the apothecary via assumed identity, fake it.)
With the remaining kill team dead, you are free to engage in disabling the gellar field and allowing for your untold warp brethren to enter the ship as it makes its way to Macragge. Upon arrival you shall lay waste to the planet before assuming the role of Roboute Guiliame, spiritual liege of the Ultrasmurfs and God Emperor of Ultramar.
With total control of Ultramar, you can then begin on your true plan, opening a chain of pizza shops.