Hi,
RPG can be played with good, neutral or evil characters, it doesn't really matter as long as whole group is following the same character path. I will write this from D&D point of view but this can be applied to any system.
There are very simple rules to keep control of any campaign:
1. Make them hate someone as much as possible (BBEG, BBGG in this case?). This is rather simple.
They want to burn village? BBEG is living there, have some business there, or he is just bigger bully then they are.
After he will kick their asses, rape them, torture for three months and make them beg and cry they will hate him enough.
If player will say "my character is not crying, he is strong", ask him to make difficult Fort save each day of torture, sooner or later they will cry.
2. Make them aware that every action is leading to reaction. This is the case for evil and good also.
They are not the strongest people in the world (if they are this is some god-like campaign).
If they are burning villages then someone will react to this. Deploy regiment of troops or some ultra heroes that will own them. If they are very evil let them die, then raise them (losing level for being raised will hurt them).
3. If they are not willing to follow your lead... this one is tricky. Try to convince at least one of them. Then focus 80% of your attention on player/s following your path, rest will have 20%. So if someone is not willing to retrieve ancient artefact and want:
"I want to burn that village and rape every one there" then don't focus much on that "Ok, village is burned and everyone raped" the go to the group that is following your lead and describe their adventure in details. Alternatively you can assume that everyone that is not following your lead in playing on "hard" level (harder opponents, bigger risk of death, less rewards etc.) while group that is following adventure hook will play at "normal" level.
4. If group is not following adventure hooks let NPC do that. If you tried to involve them in princess rescue mission and they said "fuck it, we want to do other evil things" then let them meet some time later NPC who rescued princess and in reward get shiny magic armor that they can only dream of. Make this NPC so strong that they can't just kill him and take reward for themselves.
5. NPC are for your eyes only. Don't bother to explain to them why NPC is having 200 HP. It's your business, not their.
6. As already said, this is
De
Mocracy, not democracy.
7. If they are killing your NPC don't be angry, you are having many cloning machines and can spam them with the same NPC over and over (different name, different appearance, the same stats).
Hope this helps.
Best regards,
Pelzak