The three drug dealers being skinned alive and tossed to their deaths. Being skinned alive is bad enough (we even get wince-inducing glimpse of skin being sliced off), but the use of Slo-Mo means they fell for what seemed like hours, in agonizing pain, and unable to do anything about it.
Slo-Mo only changed their *perception* of time, so the impact causing a sudden end to the nightmare would cause a momentary relief before death.
On the same note, Dredd's execution of the drug-user at the beginning; namely, firing a Hot-Shot round (read: flare) into his mouth. We get to watch his head graphically being burned from the inside out.
Ma-Ma. A hyper-violent, scarred-up, crazy-as-hell woman with a bias towards brutal murders and has absolutely no problem killing everyone in Peach Trees to kill Dredd.
Most of the death scenes have no Gory Discretion Shot at all, leaving quite a few mutilated corpses for the audience to stare at.
The few ones that do aren't much better (i.e. that poor homeless guy).
On a more mundane level, the poor kids playing in the skate park on or near level 75. The whole building is locked down, and they can't go in or out until Dredd blows the outer wall out for different reasons. Further, it's snowing and they're all wearing day clothes.
The mere fact that there's a skate park hanging precariously perched against the building on the 75th floor, over a loooong way down. No OSHA Compliance, indeed.
A militarized police force acting as judge and jury empowered to perform summery execution. Based on the world they live in paper work and due process probably isn't high on the to-do-list.