At first, Macil seems to be that type of guy who fights for justice and freedom. First, you have to complete a few missions for him (such as freeing the prisoners). After that, his forces are ready for an assault on the castle and he'll tell you to join the assault and destroy the Programmer. Once the Programmer is dead and The Front has taken control of the castle, Macil will tell you to find the other sigil pieces, and to seek the Oracle. Later, the Oracle will reveal Macil's true interests by saying that he is the one who holds the "third piece".
At this point, the player can choose to either believe the Oracle's words over trusting Macil, by killing him first (this will cause the "not-so-good" ending to happen), or the player may still trust Macil rather than the Oracle by killing the Oracle first (this causes the best ending to happen).
If the Oracle is killed first, Macil will abandon the Sigil mission and instead tell you about a factory that turns people into Acolytes. The factory is within the commons (he gives you the Brass key, so that you can enter the commons). He wants you to find the factory and shut it down.
After you have done so, Blackbird will tell you that Macil has just sent 200 people to their deaths and that she wants vengeance. You are going to face him before you can progress. It is revealed that Macil indeed has captured a sigil, and that it has driven him insane. When you confront him he is no longer the man you knew before, and a battle ensues. After he is killed, his Spectre will emerge from his corpse and you will have to use the Sigil to destroy it. Once it is dead you should get the fourth Sigil piece.
Until the Order's castle is captured, it is impossible to actually kill Macil, as he is invulnerable to damage. However, he can be killed any time after you have got two Sigil pieces: doing so will cause his Spectre to come out and attack. Macil himself has 95 Hit points when initially fought. Being possessed by a Spectre causes him to be invulnerable to damage from normal weapons: only when you have acquired two Sigil pieces or more, can you harm him.
Killing Macil can be easier if you have three Sigil pieces rather than two: the Sigil's wide spread attack when you have three pieces makes it easier to cause damage. You could also try to use the nearby Rebel Guards as temporary meat shields, though they probably won't last long.