The devil in manhwa can take many forms. It could be a traditional evil character with horns and a pitchfork, or it could be a more symbolic representation of evil or temptation. It all comes down to the imagination of the author and the message they want to convey through the story.
To survive, you should build alliances with the right characters. Also, stay away from obvious traps and don't trust everyone easily. Keep a low profile when necessary.
Hard to say. A lot would depend on the themes, art style, and how well the relationship is portrayed in the manhwa. Sometimes it could be a yes, but other times, not so much.
The reason could be anything from a romantic encounter gone wrong to a mystery that needs solving. Maybe it's part of a larger conspiracy or just a random act that leads to unexpected developments in the story.
There could be multiple explanations. Maybe it's to protect someone or achieve a greater goal. Or perhaps there's a misunderstanding that's led him to act this way.