Well, since I'm in a helping mood right now (Not very common as of late), here's some starter tips:
1) To make your target enemy finish the level:
Go into the enemy's properties by right-clicking on it, then give it an "Is Objective" value f something other than 0. Now, make sure that nothing else in your level has an "Is Objective" value and upon killing him, you'll finish the level.
2) To make it so your player will fail if they kill a citizen (or even unarmed AI), use this command in the AI's Destroy script (can be found in their properties, and editted by finding the script in the scriptbank folder):
plraddhealth=-99999
That will decrease the player's health by 99,999 if they destroy or "kill" any entity with that specific destroy script.
To learn more about scripting, you may want to read my Tutorials, which can be found in Nickydudes Hints and Tips guide (located in a sticky in the FPSC Chat Board)
Good luck, and welcome to the forums!
The one and only,
~PlystirE~