If had to show mercy on my rival list, I would attack the same 3 or 4 people (who are at my level and have 500+ members) over and over and over again. So I expanded my attack list all the way to 50 levels below me in order to get the Thug Life goals done with some exceptions for forum posters. Now that I am done, I am going after the 499 rats full time.
When someone asks me to add them in a polite manner, I have no problem with adding them even if we were at war. The ones that I refuse to add are those who spam on my wall that they wish death to my family or who commanded me to "Add them or else" like I owe them something.
Not a chance... If they have something worth robbing I take it. If tables were turned would they show mercy? Probably not.