They don't lead to bad karma, but they will likely give you the option to do something bad, at worst give you the option to do something good with no karma change, and NEVER give you good karma, and that's really the goal of evil characters. It's not to do something bad, but rather to not do good. And merc responses is the name I give to responses that are just like "I <3 caps, give me caps, I want a reward preferably in caps if you want me to save your but". These generally lead to no karma change, which means your karma changes are driven by the people you kill, the things you steal, or how much water you give those beggars.