I think that it depends on when you attack them, if you are allied to them, your reputation will go down, if you have any kind of treaties with them, your reputation is likely to go down. As for chivalry, i just release captured men, go on crusades that kind of stuff. If you kill muslims, then you might get chivalry but i cant be sure, safer just to capture and release them.

If you are attacked by an ally, or another nation, then your reputation will not start to fall if you attack back, so if you start pwning them, i dont not believe it would do anyhting to your reputation