Monday's only know appearance was from the very first teaser trailer. Only the name was visible and no outline number was showed. Monday was meant to be the squad leader who betrayed the United State Colonial Marines. That was axed when the squad gameplay was axed.
It is very possible that Monday was going to let Weyland-Yutani PMCs onboard the Sephora and then disabled the systems inside the Sulaco. The question is how long would he had lasted in the original campaign mode had the Tactical Gameplay/Squad Roles remained.