He had his hand on a gun the whole time. I felt like the director added that to make the scene more acceptable. Had she just killed a man who posed no risk maybe they feared it would be too much for audiences.
Very good catch. If you look at earlier iterations of this script...you'll find that they did a lot to humanize the judges. I wish I could remember specifics, but indeed, the scene you're describing was set up differently if I recall correctly.
I think that's why Garland said that he had planned for a sequel to explore the idea that Dredd is a fascist, because they toned it down a lot here.
That way, viewers are like, Dredd only stunned those kids, he's a GOOD guy!
The perp was her first kill, also, Anderson saw the guy had a wife and kids and that's why she hesitated, but still kills in the end. ACAB... They then meet his wife later on.