you cant kill the dwarf king in this version, but you can if you go back to one of the 40D versions or earlier where mountain homes existed. currently, non-human sites are not in adventure mode and they are waiting to be re-implemented.
the only consequence was that you would become an enemy of your civilization, and people would attack you on site. I think a new king was assigned for fortress mode, so no impact there.
You can indeed end civilizations. easiest to do on a pocket world, but no real game-impact except that they wont be there to trade with any future fortresses. since non-human sites dont exist anymore, you cant end those civilizations as you cant find the people to kill them.
becoming king might be part of the army arc. otherwise, you can attract a king to your dwarf fortress, and become the capital.
you can not raise an army YET, but you can attract increasing numbers of followers depending on your skill and fame. you can actually get pretty large groups at higher levels. i am not aware of any practical limits to it.
back in the old 2D mode you used to be able to lower the evilness of an area by killing lots and lots of the evil native fauna. I dont know if this is still a feature or not, and it isnt very practical to do in any case. Also in 2D there used to be a limit of one mega-demon per mountain range, and you could eventually kill them all. The underworld does not work that way anymore, but there are a limited number of forgotten beasts that you could eventually kill all of. demons are now unlimited, as far as i know.