No prophecies - that's an overused and poorly understood plot device if there ever was one. It also introduces an element of mysticism that doesn't sound like it would fit with your tone.
Does the protagonist have to be a man?
It might be better to put the protagonist in a Brutus role - recruited, reluctantly, into helping the conspirators in some minor way because he is convinced by them that the current leader is corrupt. I agree with Temis - the prtagonist would have to have some talent, skill or something the conspirators need, like the right position to distract some vital person who would normally protect the leader who's being targeted. That way the protagonist is involved, but not in any way that would get blood on his hands. I'm thinking some kind of counselor or lawyer.
Shortly after the murder, he discovers that the situation is not as simple as the former leader being a bad guy and these folks being liberators - but it should stay pretty ambiguous. The former leader was doing something iffy, such as making arbitrary decisions about which families would live and which would die due to limited resources. But the new leaders are either doing the exact same thing (except choosing people more to their liking to live) or doing something equally as iffy. The protagonist is now enmeshed in the power structure of a group, his family's well-being is dependent on his staying on decent terms with them, but he insists on his job with the organization being clean. They agree and set him up, as you say, to keep good relations with the public, providing services to the community. But then someone comes to him for help who has ended up on the wrong side of the current leadership...