Here is all of the information you need to get the worst (Devil) ending in the game where your Opinion Leader gets possessed by a devil.
First of all, you are going to need the Brunhild Sword from the Kastoaltian Sea. The only problem is, you have to have a reasonably high reputation to get it. This means fooling everyone until you get your hands on the sword. Basically, just play along and do everything to improve your reputation until you get your hands on this awesome blade. Once you get it, all semblance of honor and integrity can be dropped.
Your only other requirement is to have Galf in your party. To do this, you need a rock bottom reputation, you need your Opinion Leader to have less than 30 ALI, you need to have the Bunhild Sword, and you need to NOT have the character Fogel in your army. If you meet these requirements, Galf will offer to join you.
Once all of this is done, just work on maintaining a low reputation, and you should get the worst ending.
One of the biggest problems with the low road is finding good allies. Because you need a low reputation, most of the "good" special characters are going to turn you down when you find them... though it is still possible to get them (refer to the Recruit Everyone page for more information). Therefore, there aren't lot of choices when it comes to who sides with you. The easiest way to get around this is, after getting the Brunhild, create a few obscenely powerful groups and use use them constantly. A great group combination is a Lich + Princess + Sky unit. A Lich alone can wipe out most groups he comes across, the Princess adds extra insurance and White magic to the group, and the Sky unit (Birdmen classes/Gryphon classes/Wyvern classes) adds mobility. And, since you don't have to worry about your reputation anymore, so you can just let them slaughter everyone without caring about maintaining their ALI and CHA.
There are many ways to reduce your reputation. You can lose reputation by liberating towns (temples don't apply) with low ALI groups, you can allow your liberated towns/base to be taken over, you can continue to collect multiple tributes (you lose reputation at the 4th tribute offering and each subsequent tribute in the same stage), you can draw reputation reducing cards (Devil/Death/Fortune), or you can make reputation reducing decisions.
That's all there is to the low road, a far cry from the time and worry you have to put into the high road. Have fun beating it this way.