That’s not quite the point.
Étienne probably somehow knows that these people needed to die.
But he didn’t expect Rishar to do it like that. Rishar is that super sweet protective lovely person in his mind. And seeing him murder someone without second thought in anger (just like Étiennes mother would) was probably really unexpected. Not just because of the murder but because he suddenly saw some of his mothers traits in the person he’s put on such a high and shiny pedestal.
Also, the information they could provide. They needed that before killing them, cause now it's harder (pwuld they have talked, probably not? But the chances are never zero ig
Look, I understand that watching people die is traumatic. I don't think I can handle it.
But these people knew his identity and that he was an Omega. They couldn't be allowed to live. The second he chose to reveal his identity he guaranteed that they were going to die. If Rishar didn't kill them then his mother would.