How do you make something say him/her according to their key gender?
There are text macros, such as \himself:

//src = a mob
world<<"\proper \red <b>[src] has killed \himself.</b>"

This will replace "himself" with "herself" or "itself" depending on the gender. By default mobs inherit client.gender. It's possible that this is not inherited if you override things like client/New() and in some other cases. In that case, you should check this at mob/Login() as it will also allow you to change this information through Your Account.