The big downsides with horrible prison are that criminals will have a huge incentive to do anything to not get caught and convicted, including violently resisting arrest, shootouts, targeting witnesses and so on. It is mostly effective as a deterrent against financial/winning-relates crimes, but mostly ineffective against most other types of crimes. Plus, horrible prison is much less likely to actually reform the person in question.

Statistically, the Norwegian system with "holiday prisons" works well, with very low recidivism rates for most types of crime.

Norway's intention homicide rate is six times greater than Singapore's.

That’s why execution is the ideal solution, not prisons. During a brief transition period you may have to deal with violently resisting arrest and the like, but eventually you get to the point where all the criminals are dead.

本当の殺人鬼だな、キラは.

Hah! It had some flaws, but I did enjoy that manga.

Studies show that the severity of the punishment has little impact crime rates -- the odds of getting caught & punished are much more significant.