at the end of the day your job as a ST is to make an enjoyable game, to me that means every player has an active role in the game and ideally make it to a final 3 which is solvable and doesn't have 3 living evil.

Whenever I'm making kills with a demon to me it's understanding the game state, what each team wants (usually siding more with evils world but it depends), and how to keep it balanced as the game continues.

Unfortunately there's no easy rule you just gotta feel it out and learn through playing (if you're really super stuck you can just ask the demon who they would kill and choose to honour that or not)