The Judge brutality bit didn't bother me too much. Ma-Ma had just pulverized an entire floor with her mini-guns and dozens of innocent bystanders were killed. Dredd had to get information out of Kay to figure out why she went to such extremes in order to minimize further civilian casualties.
With regards to the vagrant, the Lawbook might have stated that a warning should be issued if it's a first offence. If that was the case Dredd would still be following the law to the letter.
But giving Ma-Ma a hit of Slo-Mo before chucking her off the top of the building just seemed a little bit sadistic and vengeful for a man that doesn't really harbour much in the way of emotions. And I can't imagine that the lawbook would approve of such an action either.
There was another iffy scene that was cut from the film and replaced with alternate footage. After Zwirner exists the overturned van, Dredd shoots through an innocent woman who was blocking his aim. He then tells her a medvan is on the way and she need not worry 'cause he deliberately didn't hit any vital organs.