You can't - and I mean that technically.
Quis custodiet ipsos custodes?
Who watches the Watchmen?
In order to carry out violence against those the society deems undesirable (and forcibly kidnapping someone is violence - especially if it's to administer a death sentence) - then the Dictatrix needs people who are Violent.
Let's run through the scenarios - you can detect violent tendancies at Birth - you kidnap the baby - Ever heard of a Mamma Bear? There are numerous instances of Women (and Men too, but primarily women) doing near super-human feats to protect Babies.
Okay, so maybe not Babies, maybe we detect it in children? Same problem. Mothers will tear through whoever to protect their children. Dads too.
So let's then say - we wait till they are teenagers - an average Male Teenager is physically a match for an average Adult Female - in terms of Height, Weight, Speed, Strength - which means you will need average adult Men to enforce this.
I'm focussing on Men here - because we are bigger and more aggressive on average - and so are more likely to be the problem.
In order to weed-out those who would be Aggressive, the State will need to maintain a cadre of Humans that are Aggressive. This is your technical problem.
Ever facet of your scenario has an issue - in order to train dogs to kill Humans, sure it's not very difficult... But someone has to do it and that someone needs to have the aggressive instinct to control a Dog that knows how to kill a Human
Let's go with a tech solution - this is all done by killer-AI robots - a bit more plausible - even then though - someone has to program and maintain the Robots and be have a means of controlling them so they don't turn on their masters
Which leads to the other technical problem:
The threat of force and immediate overwhelming Violence is the best deterrent against Violence.
Consider two people standing on a street corner. One is all hunched over looking around nervously, weighing 60 KG dripping wet and looks like they've never had a touch of testosterone in their life.
The other is a 6 ft something musclebound behemoth, with an LMG slung over their shoulder and a T-Shirt that says 'Fuck around and Find out'.
Which one are you more likely to try and victimize?
And the follow-up - if both of them are being mean to you - which one are you going to take a swing at and hope for the best?