Alright, in more detail: It's fairly logical of Four to get the Seers killed, quite understandable to get the treacherous would be Empress killed as well. But why kill his stepbrother? Why kill the other member of court? They acted 1) surprised by the revelations; 2) flocked to Four's side; 3) would help the continuity of power in regards to the public. There was always plenty time to get rid of them later if needed.
Fine, let's say Four just became mad and that yes, he ordered all of those people killed. Now, the guards just moments before chose to follow the orders of Four's stepbrother instead of the would be Empress. But it took quite some convincing. True, Hiro stepped aside and Four became the rightful leader as far as the guards were concerned. But they waivered in their obedience just moments prior yet kill the person (Hiro, Four's stepbrother) they chose to back with no second thoughts?!