Someone posted about setting the king flag for a unit but not using the regicide option. I checked the civ help and it says the king flag makes a unit a king with all of the advantages and disadvantages. But it doesn't say what those are.

If a unit is flaged king you start the game with one of that unit type, but if that unit type dies (in regicied) you lose. in mass regicide you get a hole lot of them, (and so do your opponents) but if all of them die you lose.