What would you do if you were Rex?
Without knowing your fate, would you have drank the coffee? Would you just walk away not knowing what happened to Saskia?
Or, would you go for a third option I thought of which is to capture and torture Raymond until he gives up Saskia's location? Rex says he doesn't want to punish Raymond, but I can certainly imagine being willing to go to jail for torturing or murdering my wife's killer. I'm not sure if this is what I'd do, but it would have entered my mind if I was in Rex's position.