She talks about justice all the time, and yet she was getting a thrill out of killing people, there is that mission where she is tasked with the capture of some criminals, and her partner for that mission is wave, but when wave caught ups with her, she has already killed them all.
Is at this point that Wave realizes that she is fucked up in the head, while the death penalty is not unheard of in their world, the result is more than likely that their crimes were too small for that, at best they would get forced labor, at worst they would lose an hand and serve as an example for others, just like thief's got deal with in our own world.
But in most cases people would not be killing peasants left and right for small crimes, because you know, you going to run out of people to work the fields very soon if you kill everyone who does a small crime, and wave himself says that the Jaegers dont have the freedom to kill whoever they please, and yet seryu was killing people in every mission, for crimes both big and small, simple because she believed herself to be "just".
If she had listened and judged those criminals like her job requires her to do, she would have discovered that the reason they were stealing food is because the lord of their area was increasing the price of the early tax to the point that they were left with nothing to eat, it was her job to discover things like this and fix it, she is supposed to protect and serve the people of the empire, not just kill whoever she pleases left and right.
At the end of the day I hate her because she is a hypocrite, Esdeath is woman enough to outright say "I like killing people, you got a problem with that?', but Seryu is a coward who hides behind her justice bullshit while having orgasms every time she kills someone.