Interrogation of Yashur:
1. Are you from the city? Yes
2. Are you a Mafia member? No
3. Are you loyal to Argat? (Are you Mafia in another way?) No
If the first statement is a lie, then he is a troll because he is not from the city and not a Mafia member.
If the second statement is true, then there is a logical error because he is Mafia, but not loyal to Argat.
If the third statement is true, then he is loyal to Argat, but not a Mafia member.
In conclusion... Execute Yashur