1 Mrs Arden: Why do you say that the murderer is always in the room?
2 Angela: It’s just one of those things about murderers. They always return to the scene of the crime.
3 Angela: They always leave behind clues. They always underestimate the investigators.
4 Mrs Arden: Sounds like they have poor judgement.
4 Angela: They’re murderers!

I initially wrote Angela's last line as, "What do you expect? They're murderers!"

But I think leaving off the "What do you expect?" makes it a bit more subtle and amusing.

I hope.