Well, Luther has no idea if Archer is telling the truth. He's getting "He died, so I got rid of him. Done." Archer isn't giving details, and he's basically acting like a shifty robot. If you didn't know him, you'd think he was hiding something. He doesn't exactly come across as a well-adjusted human being, even though he's perfectly polite. For all Luther knows, Archer killed Trenton and Isobel is traumatized because of that. There's like no evidence because Archer's sitting on it and clamming up.
So I can see where he's coming from.
Joanie says that it isn't legal, and that she greatly appreciates Sam's restraint. He will get rewarded for not beating up on her bro.