I'm not an expert on the topic of leadership over an entire country and what skills are necessary to maintain order. It seems excessive to me that she would employ torture and death to individuals who enforce the law without her authority (example being adventurers killing bandits for stealing), but I suppose I understand the point? She can't have people undermining her authority.
this is absolutely based on IRL political issues of the past. (part of why Realm of the Ice Queen is such a good RPG book, it 'Warhammer's' real period politics)
one of the biggest issues for kings and queens and governments until surprising modern times was getting people, epesalily the powerful and rural, to respect your laws and court systems instead of taking it into their own hands or using local traditions. and they would come down hard when even given the chance to show 'I make the rules, not you.'
This is a messy area with a lot of factors, but one of the big reasons for this again is the matter of
message time and
travel time in a pre-railroad, pre-telegraph era.
If it takes one week to send a message from outer Kislev to the capital, and one week to get a reply, that's a massive incentive towards decentralizing and devolving power to local communities, for all the decisions that can't wait two weeks, and a lot of decisions that can wait two weeks but people really don't want to. And "two weeks" is much more of a concern in an era of shorter, riskier lives. The edges of Kislev can be thought of as partway to being an overseas colony in terms of difficulty administering them from the capital.
And sometimes people tried to wait but didn't get a message after two weeks because the messenger was delayed, did their own local decision, and then got a reply on day 16 saying to take a course of action that was now impossible, and they had to wait another two weeks (+further messenger delays, +waiting for the right person in the capital to be available, +other trouble) to ask what should now be done instead, and so on.
At which point perhaps the Tzarina decides she has to come herself, and the Tzarina with her court and her bodyguards and her pack train travel slower than a messenger changing horses, and soon the months start racking up. So the Tzarina has to be
very clear and forceful about what her will is, if she doesn't want to be constantly riding back and forth across Kislev to resolve disputes, and stomp on local power centres, and override old habits and traditions, and so on.
Under these conditions, trying to centralize power and standardize law and make everywhere actually subject to Katarin, rather than subject to a local noble who will hopefully follow Katarin's instructions in a general sense, is a very difficult task which requires a great deal of enforcement. Katarin is being brutal partly because she's fighting not just people undermining her authority, but a power
gradient of sorts where power tends to leak away from her.