From what I have been reading on, he feeds on misery(or something close to) by offering people something they want but it bites them back in the ass completely(gambler from the video, Veigar exploring the world). Does he go researching people he thinks he can make miserable or does he actually become a "nice guy" in a sense he will help someone without making them miserable if it is at the cost of others?
Example: Pretend X wants to kill Y in order to make Z suffer but is unable to get to Y in order to kill. Y and Z are madly in love but X hates Z for taking Y from X. Tahm notices this and "helps" X kill Y in order to feed off the misery of Z, basically making Tahm and X happy at the expense of Y and Z.
Also does Tahm instigate wars? I believe there is a saying that war breeds misery so wouldn't he want to instigate war in order to constantly be full or does he want misery to be bred in different ways? Perhaps he did this before?