In combat, what specifically affects the percentage of the war monger penalty? I've got it at around 45% atm and I want to know ways of lowering it since it's making combat a pain now. I'm guessing getting the Peace Accords taken out of congress and getting Casus Belli voted in would help decrease it? I know that liberating cities helps, but there are barely any cities which have been conquered by any other civs. Is it just a lot of not declaring any war?
Additionally, if I have a city state capture an enemy city, does that count to my war monger penalty?
Global Peace Accords increases the penalty and reduces the decay rate, Casus Belli does the opposite - so yes, that would help.
Passing sanctions against a player will also reduce the penalty for DoW'ing them or capturing their cities, but does not apply retroactively.
Liberating cities reduces your warmongering penalty, yes. If there aren't any available, not much you can do besides the World Congress resolutions.
City-States capturing enemy cities does not count towards your warmongering penalties.