Basically everyone is given a role at the start of the episode but no one knows who is who. Whoever gets the most votes in the end gets the prize. However, if you are the angel, if you can convince people to vote for you and you end up winning, all these people who voted for you will get the prize too. If you are the devil and you get the votes, then you get the prize and no one else does. But there is another twist, since if you vote for whoever ends up not an angel or devil and they win, that person will get that prize and more prizes from the votes they got. So they all try convince they are angels and accuse each other as devils throughout the episode
One episode I watched, Ong ended up a devil and he convinced everyone he was an angel and the look on everyone's faces including Daniel who voted for him was so funny
Unfortunately, idols can't really fight back since it wouldn't be respectful
But I agree what you are saying the WI hosts. They can be really bad at times and now that they are on Idol Room, I'm worried if their behaviour is the same.