I know a bit of command blocks, so I could explain how some of it would work (not using commands because java and mc commands are not the same)
After the countdown timer, the script would pick a number between 1 and the amount of players.
This player would be assigned a scoreboard called murderer
The script picks another number and is assigned police.
The police is set to a team with all the survivors that has friendly fire set to false.
The murderer is set to different team so h3 can get his weapon..
The survivors though, can hit the murderer, so im not sure how to fix that.
The policeman is assigned to ANOTHER TEAM, so he can get his weapon.
The policeman gets his weapon, and also the murderer by checking for the players on the two teams.