You can´t because...well, because you can´t.
Maybe there were women in the development team who feel "uhh duh...bad" about giving players actual freedom to act and roleplay as they want because, you know, acting thinking only in your own benefict or simply being an evil person in a videogame is "uhh duh...evil", which is just another point in the large list of negative points this game has, specially compared to previous Bioware games and how well implemented they had Good, Neutral and Evil actions, choices and consequences.
You are not supposed to. I don't think your army of the faithful, who are fighting against an evil guy, would stand by and let an evil guy lead them.
"I am Andraste´s will incarnated, she saved me from the Fade in order to make her will and believe me, you (whoever opposes you) are wrong about her and The Maker. Now follow me or you, your entire family and even your fuc*ing pet will suffer eternal punishment in the afterlife"
That should be enough for any stupid andrastian of the Inquisition (which is basically 100% of the army and inteligence network and around 80% of companions) to follow you.