Jesus, why say THAT to him?

SPOILERS ALERT:

In a scene past the 1 hour and 2 minute mark approximately, that detective says to the main character the phrase to an extent of "You seem astonishingly calm, for a guy who is about to spend the next 30 years sucking unwashed duck".

I mean no offense, but Jesus God, why say THAT to him, isn't that, besides being threatening and whatnot, also incredibly immature and highly unprofessional, and might even increase the possibility of him later escaping from prison if sentenced and fund guilty?

Not to mention, he is unwittingly admitting to the major flaws and brutalities of the police and jail system in UK and speaking as if - yeah, it WILL happen to you and we will do NOTHING about it, maybe even think you deserve it etc, as if putting people into prison even for crimes committed is not about justice and reforming an individual while punishing him but about making him brutally suffer as a painful and emotional vengeance for having the audacity to commit a crime.

Was anyone else kinda SHOCKED when hearing that detective say that to him? Thanks!

The greatest trick the Devil has ever pulled was convincing the world he didn't exist!